I am pretty sure in most countries riding a bike on a normal street requires absolutely no license, verification or anything. Even a 5 year old could and would be able to ride there (if their parents permitted or for whatever reason).

So this means that bike riders do not need to know exactly what all signs mean, what are the rules of giving way (or receiving it), etc.

So, that point is absolutely valid.

I have a friend who doesn't have a car licence and cannot distinguish many signs (for instance the circular one with red border and full white inside) or when to expect to have the priority.

Not supporting the rest of the comment, just that specific statement which is a valid one.

"Not supporting the rest of the comment."

That unsupported statement is not helpful. Whether right or wrong I at least give reasons or background for my positions.