How do you learn from senior developers

There are several different nuances to consider depending on what Mr. Senior thinks. However, a general answer would be:

There is nothing you can do and you have absolutely no reason to do anything about it.

There are two possibilities: either Mr. Sr. is indeed too busy and is prioritizing some other work (contrary to Mr. Manager's request) because he knows it is the best course of action for him, or he just wants to slack off a bit. In any case, you cannot complain to Mr. Manager since he is not there and you obviously cannot force Mr. Senior to do anything. You might complain later if Mr. Manager happens to come back from vacation before your departure, but that wouldn't get Mr. Sr. to work in his absence and possibly burn bridges.

The thing is, I don't understand why you should deal with Mr. Sr. when you can do your best with your intern? You will not be responsible for the future of the project. If Mr. Senior doesn't feel like updating himself and the project suffers later, this is not your problem as long as you continue your work until the end of your internship.

Now for "workplace policy", if Mr. Manager asks about lack of progress, you can simply tell him that Mr. Sr. has been busy and has not been able to allocate you time. So you just did your best. It doesn't matter if it is true or not, you are just telling Mr. Manager what Mr. Sr. said to you without adding or suggesting otherwise. You're not complaining about what makes you look professional and officially doesn't put you in a bad position with Mr. Sr. Of course, Mr. Manager will understand that Mr. Senior has not done his part and may take action, but either to take that or to take responsibility for the lack of progress in the project, which may (or may not) adversely affect the outcome of your internship , but it could be depending on your situation).

If Mr. Senior wants to piss you off at this point, there isn't much you can do about it, and you probably shouldn't bother because the person who might be giving a lead on your internship is likely to be Mr. Manager. and not Mr. Sr.

Also, remember that you are only an intern: as you put it, you did not know Mr. Sr. before and he did not know you. He likely won't care about you and will likely forget you soon after you leave. You shouldn't think too much: if he doesn't want to help, it will be more experience for you.