I recently got back into cycling and it took me a while to achieve a good fit on my new to me second-hand bike. On the one hand it is very simple - find a bike that is your size and adjust seat and handle bars to your comfort. OTOH, we are all differently shaped with different flexibilities and different goals and riding styles. There are some basic guidelines to follow (for frame size and seat height, for example), but the fine-tuning will depend on so many different factors, there is no 'one size fits all.'

I started with the basic guidelines but was quite uncomfortable on the bike after a few kms. Through a lot of googling and trial and error I discovered that my right side is significantly tighter, i.e., less flexible than my left. I had to lower my saddle quite a bit (about 10cm I think) to be able to ride without pain. Once I did this a few other problems magically resolved themselves. In essence, I found a sweet spot for this particular bike with my particular body. So for me there was somewhat of a learning curve but I learned a lot about fit and my own body in the process and it was definitely worth it.

I guess what I am trying to say is that knowing what a good fit is for you personally will come with time and experience, trial and error.