Might want to start by knowing what kind of riding you want to do most. Road, gravel, recreational etc. I would then try to test ride all the bikes in that category from all manufacturers. Next is finding out which size fits you the closest and start from there. Comfort is king here...
Once you get to this point and purchase the bike, ride it for a few hundred miles or so and start adjusting all the contact points first. Seat height, seat for and aft, stem length, handle bar height etc. You can do these things by yourself to get the right fit for your body and comfort. If you don't want to do these things yourself this is where professional fitting will occur.
Getting the bike first in the right size is your starting point. Bike fitting will come later as you get accustomed to your new bike.