Why do I need a license in OpenSource projects?

Question

Law in IT

1) There are different licenses, some prohibit the use of the code for commercial purposes. How can it be proven that I am using licensed code?
2) Many commercial projects post code on github, mainly to demonstrate that even knowing the code, the system cannot be hacked.
For example, (perhaps not the best example) telegram is a commercial project, it just doesn't make money.
All clients and cryptography are already publicly available.
They say that they will gradually lay out all the code in open access.
This means that any richer billionaire can launch his telegram, only with a different name and invest more money in it and replace telegram.

Answer 1

There are already 3 questions, 1 in the title and 2 in the body.
This is a "crutch" in the world of copyright. If the author of any work does not specify the terms of use of the work, then in many countries, as a rule, this means that the explicit consent of the author (in writing, for example) is required for the use of his work. Therefore, they use ready-made texts of licenses that cover all legal issues and protect those who use the author's work from the claims of this very author (no one knows what he might come up with).
You can find traces of use by doing reverse engineering. It all depends on what type of application we are talking about. If it works on the server side, then the fact of using it is easier to hide.
In theory it might, but in practice it doesn't work that way. History shows that few people will leave their usual service / product to switch to some new one, even if the latter has more advantages. Users are very conservative, they do not like changes, and even more so they will not voluntarily switch from a service where there are all their contacts to a service where there is no one.