Others have already mentioned the official documentation, which is a solid starting point. If you are jumping into a existing project, chances are that the steepness of the initial learning curve will be determined more by the choice of libraries beyond core Angular.

You'll certainly have to also get familiar with the API of some ready-made component library (Material and the likes), as well as probably some sort of state management (NgRx). The latter in my experience is what needs the most time.

My recommendation: invest dedicated time into learning RxJS, for it is deeply entrenched into core Angular, and it is the basis of all more sophisticated state management libs. Don't get overwhelmed, because in practice it will boil down to 6-10 operators you'll use a lot (map, tap, filter, mergeMap, debouce, distincUntilChanged, take, combineLatest from the top of my mind) and the tail end of little operators you'll look up when needed. https://rxmarbles.com is good for visual learners.