How to change file permissions of ALL FILES included in a folder and its subfolders?

In the Finder, select the main (parent) folder. Then do GetInfo (CMD-I). Unlock the Sharing & Permissions pane. Change privileges as desired. Then click the little gear icon and select Apply to enclosed items.

Be very careful with this, as it WILL change the permissions on EVERY child folder & file.

Use the Apply to enclosed item in the … menu button. Be careful with that as it will propagate ACLs if there are any set on the topmost folder. Best to use chmod in Terminal if that is the case. Or, remove the ACL, then add it back when done, but that would require using chmod in Terminal anyway.

You should not be recursively changing permissions (or ownership) of a folder's contents where you personally did not create the folder in question. Not in Finder nor in the Terminal.

And your term "main folder" whether it is your HOME user folder or some other destination could very well require you to perform a clean reinstallation of the operating system and a restore from your Time Machine backup prior to this misadventure.

One of the participation challenges in these communities is knowing how to do something, but realizing that some shared knowledge that is misunderstood, or in the wrong hands can actually create a larger problem, and thus should not be shared. 😉

If you had worded things differently and shared that the main folder whose hierarchy you wanted to alter is located on an external drive, then my response would have been entirely different, or more likely, I would not have responded at all since it was already handled.

Enjoy your weekend as well.

👍+10 Excellent advice & caution.

For my part, I have only done this on data folders that I created myself, mostly on external drives.

Amen!