This is a real can of worms, as you can see from what other people have said. Let me chime in with two things: first, it depends on the partition and filesystem type, which usually depend on which distribution you're using. For example, Debian, Ubuntu, and Linux Mint default to the ext4 filesystem, which is easy to grow and shrink. Most Fedora derivatives use btrfs, but Fedora Server uses xfs, which cannot be shrunk. So you'll need to know what filesystem your system is running in order to proceed. The safest thing to do if you go this route is probably to backup everything and reinstall the OS, picking the new size from the installer. However, depending on the filesystem, resizing may be possible. Regardless, this is potentially dangerous and the first thing you do should definitely be to back. up. everything.

And second, if you've got a big filesystem that's mostly empty, the easiest way to solve this may be to simply copy whatever's filling up your smaller filesystem to the bigger one, rename the original, then symlink the location on the bigger filesystem to its original location on the smaller one. Then test to make sure whatever you changed didn't break anything. If everything worked, you can delete the renamed original. So for example, from the terminal, I'd do:

# Assume the smaller filesystem contains /home and the larger is at /mnt/bigdisk
$ cp -a /home/USER/.local/share/Steam /mnt/bigdisk
$ mv /home/USER/.local/share/Steam /home/USER/.local/share/Steam.orig
$ ln -s /mnt/bigdisk/Steam /home/USER/.local/share/Steam

Even though this is probably what I would do, a word of caution: don't do this if the stuff in question is required for the system to boot. And if you don't know, don't try it (or let it be a Learning Experience)! Generally, things under your home directory are safe, so if Steam's installed there, that should be fine. Good luck!

consider switching to btrfs which has subvolumes (think partitions as folders, no need to predefine their size) and you'll never have to deal with this again. also compression and deduping (all those duplicate games dlls and whatnot take up space only once)

I usually boot the System-Rescue CD (can also boot from USB) and use GPartEd to resize partitions.

You'd have to shrink the other one first, unless you left free space. If you used lvm you should be able to do this online, else I'd take it offline and boot gparted.

Although you can probably just redirect steam to the other partition, with a symlink if it doesn't let you change the dir.

If you make any partition changes, make sure you have a separate working backup of any data you can't afford to lose.

I had this issue recently. I deleted a few of the older versions of Proton that had accrued, as well as a couple of OS images I had pinned (ostree on Bazzite). Removed something like 12GB

Probably not without LVM or an external drive to restore from. Depending on which partition comes first, you'll either have to extend the OS partition "leftward" or lop off the front of the data partition, and there isn't a good way to do either nondestructively.

If you have LVM set up, you could reduce the storage partition, make a partition in the new free space, and lump it into the volume group for the OS partition.