Installing An OS On Mac Pro 1,1

Officially Apple only supports up to macOS 10.7 Lion on the Mac Pro 1,1:

https://everymac.com/systems/apple/mac_pro/specs/mac-pro-quad-2.66-specs.html

Whether it is possible to install a later version of macOS like is done with some other later Mac Pro's I don't know. Nor do I know whether BootCamp from an unsupported OS will have the necessary drivers for Windows on this older unsupported model. Since I don't use Windows or BootCamp I don't know what version of macOS is required for using Win10 with BootCamp. You would be better off posting specific questions about a Mac Pro 1,1 in the Mac Pro forum:

https://discussions.apple.com/community/desktop_computers/mac_pro

You need a retail release copy of the OSX 10.6 Snow Leopard DVD and not a gray system disc made for one specific model Mac.

How much memory does this Mac Pro have? If it has at least 2GB of RAM, then you can try installing Linux on it. However, since this is the first gen Intel model it only has a 32 bit firmware so I'm not sure if any current Linux distributions are able to handle a 32 bit bootloader and a 64 bit OS. I remember even many years ago running a 64 bit version of Linux on a system with a 32 bit firmware was difficult and today most distributions no longer have 32 bit OS versions. I know that Debian still has a 32 bit version of the OS, but I don't know if Debian will support the 32 bit bootloader for a 64 bit version of the OS. I know that the Vivaldi web browser (a Chromium based browser) still has a 32 bit version available of the current release if you are unable to install the 64 bit version of Debian.

If you try installing Linux, then you should try using one of the following Desktop Environments (aka GUI interface): KDE or MATE are the nicest looking and will work with limited memory. The default Debian download is a network installer .iso image that you can "burn" to a USB stick using Etcher (Mac/Windows/Linux). Option Boot the USB stick and select the orange icon labeled "EFI". If you are not familiar with Linux, then I suggest using the whole drive and using the suggested defaults except for the software install screen where you should de-select GNOME Desktop Environment and select KDE for the Desktop Environment (KDE looks a lot like Windows, but is much more customizable).

https://www.debian.org/

Unfortunately with Debian there is a lot more you must do to get all of the Mac hardware to work since Debian disables the non-free drivers by default which means by default the WiFi will not be functional until you enable the "non-free" and "contrib" repositories and install the proper WiFi drivers. This is definitely a great way to get a better understanding about how Linux works and how Debian handles the non-free proprietary hardware/drivers. I can assist you to get the WiFi working (assuming you have a WiFi card installed).

The EFI was designed by Apple to be 32 bit and cannot be changed. I think only Win7 is supported on this Mac using BootCamp, but I don't know if 64 bit is supported. I don't know if Win10 can be installed since the Apple hardware may require some Apple specific drivers that are only available with BootCamp and BootCamp would at most support Win7 for this system.

I would not spend the money on macOS 10.7 Lion. Lion had a severe unpatched vulnerability so you should not connect a Lion system to the Internet (we were banned from ever using Lion within our organization for this reason).

Keep in mind there are no web browsers that will work on Lion that I am aware of. At least if macOS 10.6 Snow Leopard is installed you can use the TenFourFox browser that is a PPC backported version of Firefox that should work on an Intel Mac with Rosetta on macOS 10.6, but unfortunately the developer has just announced he is no longer going to support this browser so its usefulness will be short lived. If you do want Lion, then make sure any third party apps you want to use still have older versions available that support Lion.

Besides most likely being illegal since I'm sure Apple has not authorized anyone to resell macOS on a USB stick I personally would not trust any software purchased from an unknown third party unless it is on the original media like a retail version of the Snow Leopard DVD. You have no idea if the installer is complete or whether someone has included a nasty surprise with it.

Unless you purchased a copy of Lion from the App Store you are unlikely able to access Lion today. Lion was the last paid version of macOS.

A couple years ago I saw an older article on one of the major Apple fan sites which mentioned how to acquire the installer while booted into Internet Recovery Mode so you could create your own USB installer for an outdated OS. The article was referencing an OS that was no longer linked by Apple (I think it was Mountain Lion, but the technique should work with any OS that can be accessed by Internet Recovery Mode). Unfortunately I don't recall the details although I may have it bookmarked somewhere since I was curious about it. I don't think your Mac Pro supports Internet Recovery Mode.

However, a lot of users on these forums have reported issues attempting to install Lion while booted into Internet Recovery Mode (laptops). While there may be some third party apps which try to create a bootable macOS USB installer most times I just see people complaining that those installers created with those third party apps don't work/boot (Apple made some changes to the installers a couple years ago).

Unless this Mac Pro can have its firmware updated to accept a later version of macOS, then I really think Snow Leopard is your best bet if you can find a retail DVD version of Snow Leopard (not the gray disc made for one very specific Mac). Posting your OS questions about macOS & Windows in the Mac Pro forums will get you contributors that are much more knowledgeable about the possible options available for this Mac Pro especially to know if it is possible to install a later version of macOS on it.

If you are willing to learn a new OS, then installing Linux on this system is most likely the best option to have a current OS with support for the popular web browsers (Firefox, Google Chrome, Vivaldi) plus Linux has access to lots of free open source apps. Linux definitely isn't for everyone, but it is a great way to extend the life of an old computer. I was still running an old (15+ year old) Dell 32 bit laptop up until two years ago, but I was concerned that I would no longer have a web browser. My fears were premature since Vivaldi still has a 32 bit browser available and the next version of Debian (v11, aka "Bullseye") still has a 32 bit OS available though I'm not sure how many more version will have 32 bit support (it is one of the last major Linux distributions to still have a 32 bit version).

Even if it is delivered in a .dmg archive it is highly unlikely this archive has a bootable image that can be directly "burned" or "Restored" to a USB stick. Apple doesn't like to make things that easy for their users.

Then you have to figure out how to create the USB installer from the resulting download or resulting installer app. While there may be some really old instructions for doing this, there is no guarantee the current installer is in the same format as the older version of the Lion installer in those articles. Unless you can locate recently made and confirmed instructions for creating a bootable macOS 10.7 USB stick using the very latest download you risk being unable to use the Lion installer until you can install macOS 10.6 Snow Leopard so you can then just run the Lion installer. I guess the question is are you willing to take the risk to find out?