I usually ask more senior people why they say that, and keep asking why until I get it. If this process seems to be going on too long, I ask if there's a book

I usually ask more senior people why they say that, and keep asking why until I get it. If this process seems to be going on too long, I ask if there's a book or reference I should be reading to understand the subject better. Honestly, most senior people will be very pleased that you care enough to find out why, including following up with external resources. If a senior person won't explain something if you ask politely, usually either they don't understand it themselves, or they are simply not very good at explaining things. Not all senior people are good teachers. It sounds like your team lead is one of the latter.