"____ is damaged and can't be opened."

Ok, so, I have 2 imacs. 1 is a 2009, the logic board crapped out. I have a refurbished 2012 in which the hd just crapped out. so i took the 2009 hd and put it in the 2012. got it back up and running, internet recovery, updated the os back to catalina. im currently at 10.15.7 with no updates possible via the app store.

thus far, i have had to replace the question mark items in my dock with the corresponding app from finder/applications by just dragging and deleting the ?.

then i went to download malwarebytes, and a jpg. whenever i try to open them, i get a popup stating: "____ is damaged and can't be opened."

normally, i use chrome, but was thinking perhaps it's a browser issue. safari will not open at all and sends error reports to apple, even from the applications folder.

in my gatekeeper, it also doesnt have the "anywhere" option, i had to terminal it in, but it disappears after restarting.

i've been googling and googling trying to find a solution that works, and to no avail. i have read, reinstalling the os doesnt work. ive tried other terminal codes, they dont work. ive run utilities/repair (first aid) and it doesnt work. ive run ccleaner. i also have a report via etrecheck.

i had figured i could just upgrade the os again to big sur and perhaps it would be fixed, but that update is not available, according to the app store.

please advise, cuz i dont know what else to do. i was running catalina before my hd went to crap and it was just fine. i shouldnt have to terminal anything or try to get a workaround, it should just work.

thanks in advance.