No one can hold the names and syntax of all the different aggregation stages and operators in their heads. I'd bet even MongoDB Aggregations Royalty (Asya Kamsky) couldn't, although I'm sure she would give it a good go!
The good news is there is no need for you to try to remember all the stages & operators. The MongoDB online documentation provides you with a set of excellent references here:
Additionally, if you are getting stuck with an aggregation pipeline and want some help, an active online community will almost always have the answer. So pose your questions at either:
You may be asking for just general advice. However, suppose you want to ask for help on a specific aggregation pipeline under development. In that case, you should provide a sample input document, a copy of your current pipeline code (in its JSON syntax format and not a programming language specific format) and an example of the output that you are trying to achieve. If you provide this extra information, you will have a far greater chance of receiving a timely and optimal response.