"Sorry I made you wait" means "I have been very busy, with duties that are as urgent as yours or more, and I'm sorry that you had to wait a whole day to get a response, but I'm doing my best on it".
Finding answers on your own is your top priority. Everyone is busy with work, and while part of our work is responding to less knowledgeable developers, getting a constant flow of questions gives us no time to do the job that is actually relevant to our role. If I'm a senior dev assigned with implementing X architecture, my first and foremost duty is getting that done. Replying to junior's questions is another duty, but less critical to the business and to my job, especially if you didn't state it is urgent.
Now, if you do need some answer, and X developer is too busy to help out or unresponsive, try to state urgency politely "Sorry to bother you again X, but I'm currently blocked with this issue and I would really need some help, if you are too busy I'll look for somebody else".
Then, if he is really busy, you will have to look out for another person that might be able to resolve your problem, even if it is not your "reference" mentor or developer. Most of the software projects knowledge isn't in the hands of a single person, knowledge is shared purposefully among several people. Try to reach out others who know.
Finally, if nobody can answer your question, and it is completely blocking, you will have to reach your manager, explain him the situation and either get your issue prioritised or get something else to work on in the meantime.