Many to Many- Where does data begin?

Hello! I’m using Airtable for my small nonprofit art school. The recent many to many tutorial has been helpful, but in building a database to house student registrations - I’ve hit a roadblock.

I’m following the clients to manufacturers example. I’ve built my table to be exactly the same (but with different purposes, so students, transactions, registrations, classes, departments). I’d like to build an automation from our Squarespace website so that the base populates as registrations arrive.

Can anyone answer, in the Client to Manufacturer example in the tutorial - where does the information enter? Is it through the junction table?

Thanks so much!