On Linux, not all of it, in my experience. If you have a lot of memory, some of it may be completely free.

It should gradually trend towards using the vast majority of the free space. The disk cache won't consume memory unless you're reading new, not-already-cached pages from disk; so if you're not reading enough data from disk, then it might not consume everything. (E.g., if you've rebooted recently, and just haven't read enough from disk to consume everything.)

If you work w/ removable media, IME the cache will free the pages associated w/ that media when it's removed (as the cache must be assumed invalid when/if the media is plugged back in). Playing a DVD, and then ejecting it, would often result in a large drop in disk cache. (Back before Netflix…)

Only a very small percentage is kept "free". Everything else should be made available to buffer cache or the application, etc....