Hello. I'll try to help.
To make sure I understand where you're at, when you say "I've read everything", does that mean you're read this page and the pages it links to?
If so, can you point to where in this diagram you're running into problems? If that diagram isn't appropriate, perhaps you can tell me how far you've gotten with your Rails app?
This is an accepted solution.
You manage the selling plan through the selling plan group: https://shopify.dev/api/storefront/reference/products/sellingplangroup. Look at the mutations: https://shopify.dev/api/admin/graphql/reference/products-and-collections/sellingplangroupcreate and https://shopify.dev/api/admin/graphql/reference/products-and-collections/sellingplangroupdelete and the other mutations that begin with `sellingPlanGroup`. I hope that helps. Let me know if you have more questions.