Buying online versus buying at a bike shop is an important decision, because there are some significant differences.
Buying Online
When you buy a bike online, they ship it to you in a large box. Items like the handlebars, front tire, fenders, and racks are usually not installed, so you'll have to install them yourself. Bike assembly can be completed with basic hand tools like a screwdriver and a small set of wrenches. You can buy what you need at a discount tool store like Harbor Freight for less than $30.
Most sellers provide an assembly instruction video. For example, we can look at the Ride1Up Core-5 as an example. You should watch that to see what you're getting into. The Core-5 requires you to install the front fork, stem & handlebars, crank & chanring, and pedals, which is more than some other bikes, but it's not unusual.
You'll note that the box in that video is pretty beat up. Part of buying online is recognizing that shipping ebikes involves some risk, and very few sellers offer a free option to ship a bike back even if it's damaged slightly in shipping. Rather than ship the bikes back, they usually offer an additional discount or a replacement part in the event that it's a serviceable component. Scratches on the frame are something you just have to live with if your bike experiences that kind of damage. Check the seller's terms & conditions before ordering. They will spell out your options in the event you want to ship the bike back.
The vast majority of bikes arrive without issues, but if your bike's drive system has a problem, you'll be expected to help troubleshoot. If the seller identifies that a component needs to be replaced, they'll expect you to replace the component with one that they ship you. A small number of sellers will reimburse you for bike shop expenses, but don't expect that unless it says so in the terms & conditions.
Lastly, it's worth noting that getting bike shops to work on ebikes purchased online can be difficult. Bike shops aren't terribly profitable businesses to begin with, and they see online sales as a threat to their business. They stay afloat by selling service and bikes. Many have bad attitudes toward servicing bike brands that they don't sell. Some of these reasons are justified, but many are just sour grapes.
The consequence of this is that you'll need to be prepared to do your own bicycle service. Fortunately, none of this is rocket science. If someone told me they wanted to start building their mechanical skillset, I'd suggest starting with bike maintenance. There tons of repair help articles online at Park Tool's website, and that's just one resource. Bike Gremlin is another one, and there are many more. The upside of doing your own service and repair is that you'll continue to save more money over time when compared to using a shop to do the work.
Buying at a Bike Shop
Buying at a bike shop is like using a parking valet or concierge service at a hotel. You pay more, but you get to ignore all the mechanical and troubleshooting parts of ebike ownership. Bike shops also have the benefit of being able to fit you to the bike before you buy it. When the bike is right there in front of you, you can sit on multiple bikes to get a sense for what size fits you best. Bike fitment is notoriously hard, so if you're the type of person who is sensitive to posture and comfort, a bike shop has advantages.
The bike shop will fully assemble and prep an ebike for delivery, so all you have to do is show up with money in hand. There's no assembly, and if you have any problems with the ebike, you can walk back in and it's their job to figure it out.
Routine maintenance is something you can do, or you can rely on the shop to do it. Personally, I encourage everyone to learn to do basic service, because it can be very expensive to rely on a shop to do the basics. The important part is that if you buy from a bike shop, you're virtually guaranteed to have a fall-back if you get in over your head.
It's not all roses though. Cycling tends to attract some rather... uh... fringe characters. I've walked into more than one bike shop only to walk right back out while scratching my head wondering what possesses someone to act like that. Buying from a bike shop is like being in a committed relationship. Choosing a brand/ebike is as much about finding a shop that you find tolerable as it is finding an ebike that you like. Once you find one, you have to hope that they stay in business. If you buy from a major brand (Trek, Specialized, Giant), you're pretty much guaranteed to have options though. There are five Specialized dealers within 1 hour of my house, and I live in a fairly rural area.
Overall though, the bike shop option will mean less work on your part, but more expense. That's the trade-off.