Are you new to cycling and wanting to buy a bike?

I have a few words of advice.
I've been riding for more than sixty years and bought my first decent bike in 1972, and I've gotten four custom made frames since then, so I've been around the block a few times.
I just got a new bike and am in the process of getting the fit dialed in and I have a few tips to pass on.
The chances of an "off the shelf" bike fitting you perfectly are slim. Even if you go through a fitting process, I wouldn't think of it as gospel or written in stone. Give it a chance, but if it doesn't feel right or you're uncomfortable, you aren't stuck with it. You can fix it and you should.
Before buying new parts, you can try flipping the stem over and moving the seat forward or back. Make sure the seat is level. And, of course, the seat goes up and down. If the front of my knees hurt, I might raise the seat. If my hamstrings or the back of my knee hurts, I might lower the seat.
Stems come in different lengths with varying amounts of rise or drop, seat posts come with or without set-back. Flat bars come with or without rise and varying amounts of sweep. You can shorten flat bars by cutting the ends off with a pipe cutter. In my opinion, most people should shorten their MTB bars Drop bars come in different shapes and widths.
Before you buy a new saddle, make sure it's level and straight and remember your reach and bar height affects your seat comfort. If my stem is too long, my saddle feels uncomfortable. If my bars are too low, my seat feels uncomfortable. If your butt hurts, it might not be the saddle.
Personally, I like seats that are pretty flat on top.
Whew! Hope this helps someone.