I've been fixing cell phones and replacing batteries for literally 25 years now.

On all the higher end phones that people like to have, it is absolutely not nearly that easy anymore. Not even close. The most time consuming part is if they're waterproofed/resistant and glass backed. You have to be much more careful taking the backs off without breaking (much easier with a heat gun or heating plate) and then you have to remove every little bit of the adhesive used on the back and housing of the phone if you want to seal it up well, again.

Then theres often a lot more hardware you have to disconnect and remove in order to get to the battery.

Then many batteries (looking at you, samsung) adhere the batteries down extremely hard with no release tabs so you need to use adhesive removal and thin plastic to slip under the battery (use too much or pry too hard and you'll ruin the screen) in order to remove it.

Then, if you've done all that, you just have to decide if you're going to put the phone back, back on with a sticker like what it came with (aftermarket ones aren't very good, though) or if you can lay down a nice enough bead of something like t-8000 glue. I go for the glue, because I've done it enough times that I'm confident I can make it waterproof again and I have stuff to hold pressure across the phones while it dries.