Can anyone ELI5 me how they’ve done it?
Simply put: export your data with something like Studio 3T or Compass or the mongo shell. Start a project with Atlas (M0 is free, unrestricted (unthrottled) but limited to 500 MB DB and 100 concurrent connections). Create a connection to your new Atlas MongoDB, just to keep in mind that the connection link format is different and includes all 3 instances names e.g.
Again, once you get your Atlas link right and can connect to it, you are at least 50% done.
Import, export, check your data consistency while project is connected to Atlas and pretty much done.
M0 (free) doesn’t offer backup so you’ll have to take care of that task. If you go free for a while you could check this repo and see if you can automate your backups: https://github.com/stefanprodan/mgob