Go through beaters on craigslist and youtube to teach yourself maintenance. I wouldn't buy some brand new bike in a box and try to assemble it unless it was something I'd done before. Lubing the chain and changing pedals isn't rough. Some of the other stuff...
Bicycle shops are hit or miss. Even the good ones, yeah, they'll fix your rear cassette is f-----d problem. Will ALL the gears work and will they ever shut up that you have an 8 gear rear cassette and your front is 2/3 and all the new trail bikes are 1x10/11?
No. Your bike that isn't their brand, they'll poo poo it. It'll have better components than the 3-4x as expensive new bike they try to hard sell you. They'll tell you you're buying the frame, not the components, and that the frame you're buying will last you a lifetime. Meanwhile the bike you brought in, the frame is fine, probably built at the same metal yard outside the exotic pet smuggling market in Wuhan as the one they're trying to sell you, and you want the same component they'll be selling you in 2 weeks if you buy their new bike with dogshit brakes/gears (which aren't under warranty). Pointing this out or buying their bike, then bringing in your good+ but not mint condition brakes from your old bike and asking them to just do labor might sound funny in theory, but these are bike shop people. You might find yourself soaked in 87 octane in an orange jumpsuit in a cage with that guy from Seaside Oregon who was blocking the camera and then 2v1'd the (admittedly clueless) vanlyfer, looking at you like being set on fire is kinder than you deserve.
If you do manage to kindly just get your bike fixed, they'll make it sound like they had a sled team from the iditarod to get the old part from an Inuit village, or they had to excavate a new pyramid in Egypt and lost three shop mechanics to the booby traps. Ask them exactly what the part they replaced is called, google it, then see if you can spot the first result's brand-themed box with an Amazon Prime sticker on it in their dumpster. Note, they might actually have three employee turnover in the amount of time it takes to get your sh!t back, so let that part slide.
Every person who has ever managed to tighten a quick release has worked at every bike shop in town. Every person in charge of a service department who doesn't recognize your bike, they'll try to figure out if you bought it somewhere else. This is basically like digging up their dead female relative and f-----g the body and leaving it for the carrion fowl. The person who was finishing wrenching the bike on the stand in Shop B had Shop A's brand t-shirt on and Shop A stickers not yet peeled off their bike they rode to work. When you tell Shop A you bought it at Shop B, he'll say they don't do things correctly and try to convince you your bike is a throwaway.
A week later, the mechanic at Shop A is riding your throwaway to work, having taken the same shortcut the guy from Shop B took when working on it.
The moral of the story is, when possible to practice yourself on a beater, do so. When not, dress down a bit, and as soon as they pivot to selling you something new vs fixing/changing out something beyond your mechanical comfort zone, you don't have the money.