There are three main reasons why Microsoft valued $7.5B to GitHub:-
Mocrosoft has bought GitHub as a sign to the developer community because at that time Microsoft as a protectionist monopoly is well behind them. They have the branding goodwill that they expect will come with stewarding one of the most important platforms of the modern web.
Another reason is that GitHub Enterprise is a smart but small operation. So, Microsoft can scale it far more efficiently and take it to the more large scale just by funneling their own enterprise customers. This is a win-win situation for them because it improves the value of their acquisition while making their existing customers happier.
The last reason that I think is Microsoft is going to push services like VS Code as a code editor and Azure for cloud infrastructure to GitHub customers via seamless native integration. It will lock in new developers and new clients at a marginal cost.
If you are an absolute beginner or you have a little knowledge of GitHub in that case to learn GitHub and all its concepts such as pull request or creating a repository in detail which is necessary for a programmer you can learn it by taking up the GitHub training course. If you like to read then you can have a look at the GitHub tutorial. If you want to learn it by watching the video tutorial then you can have a look at the following tutorial:-