For searching using search terms for theorems in mathlib, there is the mathlib documentation page (for Lean 3 https://leanprover-community.github.io/mathlib_docs/ and Lean 4 https://leanprover-community.github.io/mathlib4_docs/). To find theorems by type, I find the best way is to use the `library_search` tactic from inside Lean itself.