If they’re internal drives then you choose.

I like to mount drives at root, their parent directory being the logical purpose of the drive.

Got a drive you added that’s gonna be for games?

/games

Is it for movies?

/movies

Or maybe it’s just general data storage?

/data

No need to make it more complicated than it has to be.

This is standard across the industry, unless you are mounting disks that would conform to another strategy (say it’s a drive of repos, it might mounted under /usr/local/src/ as that’s where one would expect user provided source code).

I decided to simply create directories within /mnt, chmod 000 them and use them as fixed mountpoints;
for manual temporary mounts I have /mnt/a, /mnt/b, ... /mnt/f, but I never needed to use more than two of them at once.

While this setup doesn't really respect the filesystem hierarchy, I wouldn't have used /mnt at all if I were constrained by its standard purpose since having one available manual mountpoint seems pretty limiting to me.
Then again, I have 3 physical drives with ~ 10 partitions, plus one removable drive with its own dedicated mountpoint...

If I remember correctly mnt is for static media that you expect to always be present and media is for removable media which may come and go.

I myself have separate /Disks folder where I mount all my internal disks on boot. Not sure how "standard" such setup is, but it helped me keep my NTFS and Linux disks tidy and out of my way. For what I know you can mount your drives anywhere you like