I bought a bike. What next?

Yeah, I bought an ebike from Costco. It was a pretty good deal and it exceeded my expectation in just about every way, at least so far. The only thing that's bugging me right now is that the seat is supposedly super comfortable but my butt is sore after relatively short rides, but I've read that that will go away after a while.

I haven't owned a bike for decades and I was wondering if there was any less obvious things I should look out for. I already figured out that I put on the front wheel off-center because the disc was rubbing against the brake pad, but that was an easy fix. The manual for my bike is painfully lacking in details; half of covers how to use the controller, and the other half is assembly, and even that part wasn't terribly detailed. There were two parts that came in the box that it didn't even mention (a reflector for the handlebars and the derailleur protector - the latter of which I only figured out what it was after I did a reverse image search for it). It appears that it was written for people who already know a lot about bikes. The fork has shocks on it that appear to be adjustable but I don't know how or even why you would want to adjust them. Is there a website or a book that I can find with more detailed info on bikes generally?

Finally, accessories. I already bought two simple things just on impulse: a simple cheap cable lock and a bell (which seems like a strange omission for a bike, but what do I know?). I figure that I should have probably invested in a better lock, though, since it's a fairly expensive machine, so I thought I'd ask what people would recommend. Also are there any other accessories I might consider? I want to get a bottle holder, but I'm looking for a design to 3D print if anyone has any recommendations. It'd be nice to finally bust out my roll of ASA for it.