Claude Code is "agentic". Aider isn't. It can plan, use external tools, run the compiler, tests, linters, etc. You can do some of it with Aider, too, but Claude is more independent. The downside is that it can get very expensive, very fast.
In addition, after a 3 hour session I told it to create a CLAUDE.md that would help it program similarly to me, based on my preferences. I then edited that file a bit, and that has helped a lot.
Use the @ thing to prod it read some relevant files for context (kind of like with Aider)
I find it helps to have a CLAUDE.md file with instructions and thorough documentation. This is on a ~30k LOC Python codebase with type-checking and tests. YMMV with other languages.