How do I remove Chromium

Use the manual instructions. Do not download any programs.

https://nektony.com/blog/how-to-remove-chromium-from-mac

https://macflypro.com/blog/uninstall-chromium-on-mac.html

Where do you usually download files ?

Go there and if not installed just Delete them.

Not sure from where you were downloading but you should not get the

( yahoo ) search browser and something called PDF MacMaster

Suggest your run the malwarebytes app right now to detect bugs you got.