How? Add apps, install a different launcher (I've always put
Nova Launcher on my phones, as one of the first things I do after setting them up), different keyboard, etc. I haven't seen any custom ROMs for the Pixels, but some of them (custom ROMs) do things like give you control over what appears at the top of the screen (like moving the time from the left end to the right end), giving you a kernel that lets you set the aggressiveness of the speed change (the CPU changes speed depending on load - the faster it runs, the more power it uses), etc.
The vast majority of people don't do a thing to their phones except, possibly, adding a few apps, but a few people modify Android phones, not just Pixels, so much that about all that remains the same is the hardware. That's the thing about Android - it's entirely up to you. (My phone goes into speaker mode if I'm not holding it to my ear, or if I take it from my ear - automatically. Because I like it that way. (If I recall, the phone has to be rooted to do that - I think I used
Tasker to do it.)