I want to assemble my first bike, any advice on what to look out for?

Good day people of reddit,

I have not too recently ago grown a liking to bicycling, I stopped using the train and instead went with the bicycle to- and from school every day.

However, recently it happened that I hit something in the road, and fell face first from my bike, luckily I made it out with only a few bruises, however, my previous bicycle wasn't as lucky as me.

On top of that, my bicycle was never really too comfortable or adapted to my needs, I already previously fancied the idea of making my own, which is suited to my needs, however, I have no clue where to even start.

My idea is to assemble a mountain bike, and I don't worry too much about the cost either.

My questions are as follows:

1. Are there any specific sizes of specific parts? Which would make it that I can't just assemble every part which I buy, with each other? Or is there some sort of industry standard at the intersections which connect the different parts of a bicycle?

2. Which tires would you recommend using? Or rather, what are the differences between tires? Is there any importance to their size or the pattern?

3. What would each of the parts I need to acquire be called? I of course know of the rough definitions like ''tire'' or ''seat'' but when it comes to most of the bike, I am probably lost.

4. Any recommendations for a bike that is ready for urban environment?

5. Any specific website that is known, or company, which sells qualitative parts?

6. How would I measure everything, in other words: how do I make sure that the bike is the right size for me?

7. Which parts of the bicycle are legally required, for the interaction with traffic?

These are the only questions that are fluctuating in my head right now, thanks for any possible future answers in advance.