Can you elaborate on what the difference is between Hydrogen and a custom theme? Other than the tooling and template language I’m struggling to understand the conceptual differences.

I am a backend developer and lightly familiar with what React bring in frontend development. I have not familiarity with Nextjs.

Given what I know, where does Hydrogen fits into the picture? Why shouldn't I stick to sever side rendered pages when developing Shopify app?

Thanks!