Learning how to build a bike: Is it better to buy a used bike and disassemble it? As a summer project I decided to learn how to build a bike (from the comfort of my room). I have very little experience with bikes (I just ride a rented bike daily) and have no knowledge with fixing things in general (why I want to learn).
So I started looking into ways to build a bike in my room. I don't have a super tight budget (~2/3k). I'd be doing it mainly for the actual learning experience, and also of course later to get rid of my rental and ride it in the city and do some touring with it. But mainly because I want to learn about building bikes, all the little details in order to customize it and how to improve my ride; I want to be able to look at a bike and eventually feel comfortable with understanding how everything works.
After a bit of research, as I was about to buy my first pieces online, I realized that in my local bike shop there are quite a few used bikes selling for as little as 60 euro.
So I thought: maybe it's better to buy a used bike and disassemble it instead?
What are your thoughts on the subject? Should I buy a used and disassemble it, or start from scratch?
Note: consider my main drive here is learning and not quality of build. Of course I'd want it for also to be functional, but like I said I don't have the necessity for a bike to be used asap (I rent one for my daily use).
EDIT
Thanks everyone for the invaluable comments. When I posted a question I decided to buy a quite broken bike from a local shop:

Apart from the back-wheel spokes being completely broken, the hub of the back wheel and the bottom bracket where completely in need an over-haul. The bearings were toasted. Thanks to the RJ bike guy videos on youtube I disassembled it completely and got it up and running and is now my ever-day bike:
Messing with a used bike will teach you a lot (mostly through mistakes, but that's how the process works). There definitely is a huge advantage of having hands on experience figuring out how to adjust brakes, derailleurs etc.
The disadvantage of working on an old, cheap bike is that the components will be old designs and standards which does not give you experience with more modern stuff. I think if you combine working on an old bike with watching YouTube videos and reading articles you'll learn about newer standards as well.
The trouble with buying individual components is you have to know what you are doing to make sure you order parts that are compatible with each other, and you can make expensive mistakes. Also, constructing a bike from components is quite expensive relative to buying a pre-built bike.
One thing to consider, if you have a bike for wrenching and learning on, expect it to be out of service some of the time, so you'll have to fall back to rental bikes at times.
Online bike wrenching resources to get you started:
Sheldon Brown Classic source of bike component information, covers older standards comprehensively.
Park Tool Repair Help Very professional, clear and comprehensive guides to [dis]assembly and adjustment
RJ The Bike Guy Just a guy wrenching is his garage, but he knows what he's doing and he tends to get into older bikes and components.
Shimano Product Information Shimano provides extensive and detailed info and specs about their products (including older products going back to 2004). This will look very daunting and first but will begin to make sense as you learn.
And of course https://bicycles.stackexchange.com/ :-)