How can I work with a non-communicative technical lead?

I'm an L3 and they are a critical L5 so the team will never stand up for me...

First, I would challenge this assumption. It's true that an L5 will have much more output than an L3, but it doesn't mean that the L3 isn't valuable. Every member of a team is valuable and should be respected. Every L3 can blossom into an L6, L7, or higher someday, maybe even out-pacing the L5! As long as you feel down on yourself like this, you won't have the confidence to have the crucial conversations to fix this situation.

What should I do to help myself and navigate this situation?

1. Give feedback to the L5 - This is the most direct option, and you should try this if you feel like you have a great relationship with them. My "trick" here is to be a bit self-deprecating and try the classic "It's not you, it's me". You can say something like, "I always appreciate your help, but for the future, can you break things down more and go through material more slowly? I'm just starting my career, so I'm nowhere near your level yet (though I aspire to be there someday) - The extra breakdown would help a lot!". I also made an entire video on this topic, which you can watch here: Effective Communication Guide [Part 5] - Giving Feedback
2. Talk to your manager - Again, this requires a good relationship with your manager. Do this if you feel like you have a closer relationship to your manager than you do the L5. Use the same tactics as before: Come off as constructive as possible and don't sound like you're throwing the L5 under the bus.
3. Ask more questions - My rule of thumb: If you don't get something that's being said, ask questions until you do get it. I firmly believe that if you're tactful enough about asking questions and are extremely polite, you have everything to gain and nothing to lose. Here's a wonderful tactical explainer we have on how to ask for help: "How do you overcome the fear of asking stupid questions / bothering people when you need help?"

Lastly, I just want to say that it's so important to assume good intent and have empathy in this situation. The gap between an L5 and an L3 is huge. There is so much behavior difference between these 2 levels. It is very easy for an L5 to speak in a complex, esoteric way that is way too hard for the L3 to understand - Their brains are literally wired differently at this point!

In fact, that L5 was often me: I tend to talk fast and forget that I'm referring to abstractions that others don't know about, especially junior engineers. But I want to get better! I don't want folks to listen to me with a confused look on their face all the time. However, you need to tell me that you're having trouble - I can't read minds 😅