How do you make sure your code base is clean and architecture makes sense?

I’ve built some pretty simple webs and apps, but I’m planning on working on something more complex (3 repos probably: web frontend, app, and backend).
Working on single repo webs and apps untill now i’ve simply tested heavily before deployment, but i feel i can do better.
What would you say are best practices for more complex vibe coded projects?
Any suggestions will be appreciated!