I like the why behind Matestack:

Implementing two separate systems (backend-api, frontend-app) is a pain: Two different code bases, two repositories to maintain, two different deployment schedules, two test environments, two everything… Being a small dev team, we decided not to adopt this modern web development complexity and decided to create… Matestack!

If you have 30 minutes and want an easy button to learn all about it, Jonas Jabari gave a talk on it at Ruby Unconf 2019.