I think of inner & outer joins like AND & OR. An inner join will only return a result if the value of key that the join is on exists in both tables. An outer join will give rows for all key values in both tables even if values are only in one or the other table with null results in the row for the columns of the opposite table. Inner joins are the default as you are generally interested in what matches between tables but you have the option of outer if you need to get the full messy picture or if you are doing ETL and want to ensure no badish records are missing.

I think of a Lambda like you are creating a quick on the spot function/method to do something for you or for everything you have.

Lambdas are sorta just functions that you write inline and use as needed (at least in C++ land)

I have watched hours of youtube, heard hundreds of metaphors, I've read countless articles and followed even more tutorials. I am beginning to think grokking the Monad is beyond me.