Hi Korrin,
While your question isn’t really RPi specific, the concept from what I read is that you want to use serial comms (to an Arduino in this case) to exchange data with a Xojo app, running on RPi.
There are a billion entries in the forum that talk about serial port comms - and there are great sample projects that came with the Xojo app (look for Example Projects > Communication > Serial, in the Xojo folder).
I’m not always the fastest responder on the forum, but I’ll do my best to help you out.
Check out the Serial examples and post where you get stuck.
Hoping to help.