Often, when I see a handgun that interests me, I'm curious to see how its size and features compare with one I'm already familiar with. Usually, I first go to YouTube to see if someone's already done a video comparison between the two. When I can't find the specific comparison I'm looking for, I've found these sites to be helpful:
https://www.handgunhero.com/
Allows you to choose two handguns for a side-to-side comparison, including a brief summary of manufacturer's specs. Good if you want a quick look without having to skip through a 10 to 20 minute video to find the parts you want.
https://www.genitron.com/
Handgun information resource that has specs, pricing, articles, news and links to other firearm related websites. No side-by-side comparison feature, but a good place to quickly check for info on a particular gun.