Are we talking about this preload?
https://manpages.ubuntu.com/manpages/jammy/man8/preload.8.html
Cause if so, it doesn't actually let you manually add specific files to RAM. It's an adaptive daemon that automatically learns which files your applications use frequently over time and prefetches them. So when you launch it and then play games, it's observing patterns and making predictions.
This also explains why there's no "remove files" command. The files preload loads into RAM aren't locked there; they're in the page cache, which the kernel manages freely. If something else needs that memory, the kernel will evict those cached files automatically. Killing preload via htop should not really do anything, except it not doing it's thing anymore.