A Microsoft file hosting and synchronization service.
I know this was months ago, so this can't help you now, but for others who come here with the same issue - if you can just find *somebody* with an old mac, all they have to do is literally send you the "app" file itself. Not even an install dmg. Just drag it from the Applications folder to a shared drive/folder to get it to you. From there, you drag and drop it in your Applications folder and it works automatically like an installed program. I did this on a 2012 MacBook Pro running 10.15 Catalina Mac OS just the other day. My husband and I both have old MacBooks we still use, and I wanted my husband to use OneDrive but he had never installed it before on that computer so he couldn't use it. Then I thought I'd try just copying the App from my old MacBook to his, and it worked. Best of luck to all of you out there trying to preserve your perfectly good old computers.