This is an excellent thread with a large body of information for new bicyclists, irrespective of their age; thank you for the effort and time.
A comment/suggestion to new or returning bicyclist after several decades, especially the one’s who may have some budget constraints. You do not have to buy a new bicycle. Look for used bicycles in your local area, advertised by individual or check used bicycle stores, especially the kind that are non-profit - they are often doing it as a community service, they train younger people to become good bicycle mechanics so they can support themselves. They also help older riders in finding a suitable bicycle for their needs. There is often at least one experienced enough person who would actually do all the necessary measurements for you that would be enough for a custom fitting a bicycle. This service at most conventional new bicycle places would cost around $100-200.
Not any saddle is the right saddle for everyone. In community bicycle recycling shops, you will have the advantage of swapping saddles out until you find the one that fits you.
My most recent measurements for custom fitting a bicycle were done at my hospital (sports medicine) which I visited about having some shoulder problem during swimming. The physician asked me about my other activities - when bicycling came up, he asked me to bring the bicycle so they can do all the appropriate adjustments, if needed. I think someone else has already mentioned custom fitting a bicycle in this thread. It is very useful information to have - one doesn’t need get a custom frame made, one can always buy one that is closest to your needs and make a few adjustments so it becomes more comfortable.