Any coding workflow advice for existent codebase?

Hi guys,

I'm struggling to find a simple but scalable workflow for working on an existing codebase. And I can't find any tutorial or guide, or video that help me on these doubts.

1. Do you use coding agents inside IDE or with CLI? Bacause with cli you can load the agent inside a specific service, but beside this I don't see any other advantage except that maybe for CLI like opencode or Pi you don't have a specific extension for IDE.
2. I'm using copilot student, but I think I will move to Zen Go or Openrouter (I don't know yet, claude is pricy for me), and I don't understanstand how to use copilot-istructions and if it's needed. I prefere a more standard workflow to be used with different agents.
3. About AGENTS, how to build it to be efficient and not too loud? Actually I made this structure 1 AGENTS and ARCHITECTURE for an overview of the entire project and then every service has its own ARCHITECTURE and a simple AGENTS that just tell the AI to look at ARCHITECTURE, is this ok? I can improve this?
4. when you want to make a task what is you workflow?

Thank you so much