When would you use LINQ Join, ToLookup, a Dictionary, or repeated Where scans to combine in-memory collections?

In production I estimate both sides and the number of probes. If thousands of parents each scan thousands of children, an index changes the path from accidental quadratic work to roughly linear build plus lookup. I choose key equality explicitly, avoid rebuilding the lookup inside a loop, and define duplicate and missing-key behavior before coding. For data that belongs in a database, I usually let the database join and project bounded results rather than loading both tables into memory. I verify output cardinality because many-to-many matches can expand unexpectedly even with an efficient join. Ordering is a separate requirement and should not be inferred from hash-based storage.