The ForeRunner 645 Music does NOT have a touchscreen, leaving all the navigation to the 5 physical buttons on the side of the watch.
Before getting into the details, here’s my general opinion of physical buttons vs touchscreen.
In my opinion a performance-oriented watch needs physical buttons. They allow you to operate the watch without even looking at it, which is especially useful while running. Touchscreens start to become tough to operate when it’s raining or simply when you are sweating.
5 buttons might seem overkill, but I actually find that in this watch is a perfect number and after getting used to it, they are quite intuitive.
Here’s a quick overview of the buttons:

Details aside, the physical buttons are one of the main differentiators between this watch and the VivoActive 3 Music, or actually between a watch that is specific to running and a smartwatch/activity tracker.
While it might be difficult at first to get used to all the buttons and their functions, they become quite natural after a while (took me a week before being able to do stuff without having to think too much about it) and right now after two months of use I can just go to wathever menu I need almost without looking at the screen.
I cannot stress enough how much better physical buttons are for serious sport activity. I can cycle through the screens without fear of inadvertently navigating out of the menu or pausing something… especially when raining, or swim training they are so much better than touchscreen.