iMac is freezing frequently

Start with this if IntelMac...

Safe Boot, (holding Shift key down at startup), does the problem occur in Safe Mode? Could take 10 minutes or more.

Safe mode attempts to repair Disks & clears lots of caches & loads safe Drivers, & prevents loading of 3rd party extensions, so if Safe Mode works try again in regular boot.

To find out if it's system wide or user specific, try this...

Open System Preferences>Users & Groups, unlock the lock, click on the little plus icon, make a new admin account, log out & into the new account.

Does it work in the new account?

If you're using a wireless KB & Mouse you'll likely have to use a wired USB KB & Mouse, a cheap set will work.

I’d get a cheap USB KB & Mouse...

https://www.amazon.com/dp/B00B7GV802?

Windows Win key = Mac CMD key or ⌘ 

Windows alt key = Mac Option key or ⌥, also called alt key on Mac sometimes.