Is the drive available for use if Time Machine is stored on it?

Question

macOS

Let's imagine the situation: there is a poppy with 256 gigs of SSD, when you connect an external disk (1 TB), we agree to a time machine, this disk is selected as storage.
According to the information that I found on the Internet, it is desirable for the time machine to have twice as much space as the disk on which the macos is installed. Those. in the case of 256, this is 512 gigabytes.
A few questions: how is it represented on the disk, as a folder? If a folder, does it have a fixed size? Or like with dynamic hard disks in virtual machines, fill up as needed? Filesystem: APFS only? Or will the disk be simply locked, and nothing more can be written to it?

And another theoretical question: is it possible to use one large capacious disk for different time machines from different poppies, inserting it every couple of days for backup?

Thank you!

Answer 1

> A few questions: how is it represented on the disk, as a folder?
Yes, the main folder, it has subfolders for each PC, inside the directory structure is like on your poppy.
>If a folder, then it has a fixed size?
The size is not fixed, the first backup will simply make a full copy of the disk, all subsequent backups are diff.
> Or as with dynamic hard disks in virtual machines, it is filled as needed?
Subsequent backups are equal to diff
>File system: APFS only? Or will the disk be simply locked, and nothing more can be written to it?
Only apfs, you can create your own folders, but not inside the time machine directory.