How to upgrade from El Capitan to Big Sur on an 2015 IMac with Final Cut Pro

Never do this sort of big upgrade without solid backups.

The last version of FCP that works in El Capitan is probably 10.3.4. The current version is 10.5.2. Your libraries will need to be updated, as will probably most of your software.

It is best to have two backups at least: a Time Machine and a full bootable clone, on separate drives, then erase and install and ONLY migrate the user accounts. This ensures that you do not bring back lots of cruft and old extensions etc that may not work in the new OS.

Most of the problems people have after upgrading are precisely due to old and incompatible system extensions.

How long will what take? A full system wipe and clean install? Depending on your system and drive speeds and the amount of content, it will likely take the better part or most of a day. That's usually what it takes me.

Yes. The applications will all probably need to be updated, certainly FCP will. Which version are you using now? FCP content from prior to version 10.1 can no longer be updated.