Aider has a "token budget" for the repository map (--map-tokens, default of 1k). It analyzes the AST of all the code in the repo, the call graph, etc... and uses a graph optimization algorithm to select the most relevant parts of the repo map that will fit in the budget.
There's some more detail in the recent writeup about the new tree-sitter based repo map that was linked in my comment above.
There's some more detail in the recent writeup about the new tree-sitter based repo map that was linked in my comment above.