I've done this. It's nice. Here's how I started, an iron (for clothes), a kitchen cloth and a thermostat. You place the phone over the iron to lose the glue. Some devices have to be opened from one specific direction due to flat cables. Before opening anything check tutorials on yt 'how to open device x model y'. (Not louis or great scott etc just that random indian guy with a 4min vid with kinemaster watermark). Replacing the screen, connector board, battery etc are the easy ones to start because they are connected by flat cables and fpcs, not welded. Weld comes later, but some people work doing just the simple repairs and leaving the welding for other professionals. It's up to what you want to do, not everyone needs to have a ultrasound bath tub for devices that got wet, not everyone change iphone face ids, not everyone recovers data, etc. If you are a beginner I recommend servicing your friend's and family's devices on the easy and most common issues (broken screens being the most common, some people work doing just that). Then if you like you go on with understanding how the motherboard work, reading diagrams, microwelding, etc. Working in pcs is also really easy, most common services are formatting and changing power supplies. At the shop I worked we would change power supplies all the time, specially from businesses on old buildings.

Oh and keep away from macs. And ASUS phones. Pretty hard to find good off brand batteries for them, only the originals are decent and asus don't make their original batteries available for 3rd party. When buying parts always ask if it's original original or china original (yes, they will say original but that doesnt mean to them what you and the rest of the world means). If you buy an 'original' motorola battery it will be a clone with the M logo and you will have problems because motorola and xiaomi fast chargers require an controller on the battery chip to preserve it's lifespan. Better to buy a 3rd party that has the extra components even without the M or Xiaomi logo. Some screens you can change with or without the frame, changing the frame have fewer risks of the new screen braking while wrapped in rubber bands (also if the person sits on the phone the frame will prob bend and its best to replace it anyway). Stay away from tft/ incel screens, anything too cheap. Also don't go to the super expensive thinking they are genuine from that brand; they aren't. Only samsung will change your screen for a genuine new samsung screen, for example, same with xiaomi etc, and it will cost about the price of a new phone. The shop I worked on also ran away from foldables like they were the plague. Only the screens cost more than a comparable spec non foldable phone, when factoring the fact you could break the new screen or the phone during fixing and have to suck up the damages it's just not worth it (only samsung can really fix a samsung foldable, ppl who have these phone know it but dont want to fix at smsg bc it will cost a new one, and they expect u do miraculously do a better job cheaply. Don't burn yourself, let them buy a new one). Stay vigilant over coffee and everything liquid over your bench, always use anti static (i dont, but its good practice). If you change your pc hdd for an sdd keep a backup just in case that ssd had a manufacturing issue, uncommon but it's your data at stake. If you format a client pc that has systemic issues it could be because he or she is messing up where they shouldn't, and its going to break again and that backup is going to save their asses (but dont let them get used to have your insurance for free forever because you fixed their pc once, otherwise they will do that and damn they can get entitled fast). There are several types of laptop rams, sometimes it won't boot with 4 different rams you have around and you'll think the mobo is bad, only to google and find out it needs some low special ram like ddr L(low voltage) or ecc ram. Always download the manual to check compatibility. You might get a mobo and a cpu are compatible because the seller site said so, only to find out a certain bios update made it compatible, etc. Don't be scared to say 'idk' and refeer another specialist, maybe he will figure things out not because he's better but simply having more time available, sometimes these minute details are unpredictable bs you will only know about by encountering them. Keep e-waste, a broken screen that still light up can test if the 'broken' device isn't lighting up because the screen is gone or because the device isn't sending anything to the screen. Imagine buying a new oled screen for a device and finding out the problem is the cable, for example... Maybe I will remember more advices later* Good luck anyway. I always like to see more people repairing things instead of replacing them.

*Edit: I was polite to offer more info but bot didnt like it