My dock keeps shrinking and expanding all by itself every second

It looks like as if there’s an app that opens and crashes instantly in a loop which causes the dock to expand and shrink continually in the video you posted.

To test that theory, if you restart your Mac by following the steps under “Temporarily prevent items from opening automatically when you log in” in https://support.apple.com/guide/mac-help/open-items-automatically-when-you-log-in-mh15189/mac, does the problem reproduce?

Run the Terminal Command below

killall Dock

If still having issues >>

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Does the issue present in this mode ?

3 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.

4 - If not - there could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.

5 - If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.

6 - If after performing each of the above steps is the order that have been presents  and still have issues - please advise for possible further assistance.