I set up a partition/ext4 of a nvme. Even though it shows as mounted, i dont think its actually mounted. I have ] and i check and it shows as mounted. Whatever files i write to in /mnt/data it remains whether or not the m.2 is mounted or not.

I also tired to change permissions to the partition with , but it doesnt work. i still have to sudo my cmds.

thanks in advance.

Use df or df -h to see what's mounted where, and space consumed and used. You should there be able to notice changes when you add or remove files.

For an internal device, such as nvme, it's usually best to mount via an fstab entry. If you don't wish it mounted always, use a noauto option in the fstab entry. Mounting and unmounting can then be done with a simpler command, and with greater reliability.

Changing permissions one does to directories or regular files, not device files. So, you would apply the command to /mnt/data/, not /dev/disk/by-uuid/yada...

Does it matter if its mounted or not when I change permissions to the folder?

Why not try chmod while unmounted, then mount, then give it a try?

The above code is an example of the entry you could put in /etc/fstab, as already mentioned, you can add "noauto" to the options highlighted in blue so it is not auto-mounted upon bootup.
To mount manually with user read/write access, mount with the same options as in the example below. This assumes your user UID and GID are 1000 which is the typical ID values for first user created:
Umask example/meaning

FYI:
uid, gid and umask mount options only apply to filesystems that do not support native linux permissions like FAT and NTFS. They are not valid options for ext filesystems.

What mrmazda is trying to explain (I believe) is that permissions apply to the filesystem not the mount point.

Good, didn't know that, TX.
EDIT: I do remember fighting with this in the past also, gave up and just expanded the / filesystem.