You ought to be able to upgrade that Mac to Monterey. To do that read How to download and install macOS - Apple Support. Scroll down to Monterey 12. Be sure to create a Time Machine backup first, as that Support document states in Update macOS on Mac - Apple Support:
Do that first. If you don't back up your Mac, it means you don't care about what's on it and if everything on it were to become irretrievably lost for all eternity you're totally ok with that. Some people are.
After you upgrade that Mac, the easiest way to get files to appear on the iPad is to use iCloud Drive: Set up iCloud Drive on all your devices - Apple Support. I am reasonably certain you need to upgrade your Mac to something later than Sierra to be able to use iCloud Drive. Sierra is quite old.
Consider creating a "bootable USB installer" before upgrading your Mac: Create a bootable installer for macOS - Apple Support. There are a lot of good reasons for doing that, and if you would like some of them by all means ask.