Remove your expectations that you “should already know this.” Don’t try to berate your knowledge into existence, and don’t lay guilt on yourself for not already having some ultra-specific piece of information. Your assumptions about what other people in similar situations “already know” are totally wrong and based on all sorts of internal biases.
Instead, just learn it like a kindergartener. You don’t know a thing? Cool, ask someone. If they don’t know, get a book – look for words like “for beginners,” or “for dummies.” Use the power of the vast and inscrutable internet to find specific answers to questions, then read books for the general case.
Watch videos. Take notes.
This is all stuff you know. You know how to learn, you’ve done it a thousand times. But somewhere along the way, you accidentally listened to some voice telling you that you shouldn’t have to anymore, that you should just know this stuff, and be embarrassed if you don’t.
That voice doesn’t know anything. If you learn to ignore it, you can learn anything.