It definitely helps to heat the phone for a bit to loosen up the gasket. We have a blue heat mat that helps with this, as do most repair shops.
I had to go back and look since my knowledge dates back to around the iPhone 4/4S (we've been running our shops since 2014.) Only the original iPhone was glued together. The gaskets started with the iPhone 6s, to my memory. The original gasket is peelable, so it typically gets destroyed when you open the phone, but they are cheap and pretty easy to replace. If you buy a DIY replacement iPhone screen, it will usually come with it.
I had to go back and look since my knowledge dates back to around the iPhone 4/4S (we've been running our shops since 2014.) Only the original iPhone was glued together. The gaskets started with the iPhone 6s, to my memory. The original gasket is peelable, so it typically gets destroyed when you open the phone, but they are cheap and pretty easy to replace. If you buy a DIY replacement iPhone screen, it will usually come with it.