„Other“ eats up 715GB on my 2TB SSD

They are not partitions. They are Volumes of an APFS Container (like a partition).

The other three Volumes are VM, Preboot, and Recovery.

Volumes all share the same storage space with every other Volume in the Container.

Since all of the Storage Displays and helpers in macOS are almost always wrong, you should use a tool that works if you wish to determine where your storage is being used. Other is merely what Spotlight cannot identify. It could be some of your Documents, Movies, Photos, or anything else that is your data, but when Spotlight scanned to make that display, it failed to recognize them as what they really are.

I like GrandPerspective because it shows where storage is used in graphical rectangles which you can drill down into.

OmniDiskSweeper groups storage into logical locations on the disk, but does show where large amounts are used, and you can drill down into those like with GP. There are several others.

What may be happening is that those are "phantom" mount points. So, in essence, they are just folders.

The drive that was linked to those mount points has vanished, but the mount point remains. It's not supposed to happen, but it does sometimes.

If you use Go > Computer in the Finder, you should see them listed in that window.

If not, use Go to Folder and type in /Volumes

Since the OS kept writing to the mount point after the disk was lost, you might want to peruse the contents to see if there is anything in those you might not have somewhere else.

In addition to what Barney said: one common cause of large “Other” is Time Machine local snapshots.

Tell us the output of the following command in Terminal:

tmutil listlocalsnapshots /

I don't know why they are not showing in Disk Utility, but try selecting Show all Devices from the View popup menu.