What is /dev/loopx?

/dev/loop* are loop devices making plain files accessible as block devices. They have nothing to do with RAM occupation. They are typically used for mounting disk images, in your case apparently for Ubuntu Snap. See this Wikipedia article for details.

Also note that it is a Good Thing(TM) your RAM is full. Unused RAM is wasted RAM, so Linux makes an effort to put all of your RAM to good use. See this info page for details.

That is normal. /dev/loopX are virtual devices to mount image files, these are -read only- so do not get larger or smaller than they are when created.

Those mount points are connected to the snapd service. You will see extra loop devices added for every software you install using "snap". It can be removed by uninstalling the app via snapd.