The GA4 cheatsheet is the easiest way to find and look up every default GA4 dimension, metric and event.
Each variable is broken down to the most detailed level including backend name, frontend name, API name, whether it is available in Looker Studio, if it can be used in filters or comparisons — you get the picture 🙂
It’s amazing how detailed this cheatsheet gets. Plus it is updated on a regular basis to keep up to date with all of GA4s updates.