How to best utilize 1:1 sessions with Tech Lead?

First, I want to say that it's crucial to remember that every relationship is a 2-way street. Even though you're a junior engineer, strive to return the favor and add value to your mentors as well. Give deep thanks, see if you can offload their less complex work (i.e. what's less complex for a senior engineer is still quite complex for you), and be extremely respectful of their time. The best and most fruitful relationships are those where both sides feel like they're getting a lot of benefit, and anybody can be very valuable, even if they're earlier-in-career. I talk about this in much more depth here: "Dynamics of 1-on-1 Mentorship"

That being said, I'm so happy that your team is so wonderfully supportive! Here are the resources I recommend to really optimize your mentee <-> mentor relationships:

• "How do you get the most out of mentorship?"
• How To Actually Learn Software Engineer Skills
• 1 on 1s Are The Time To Be Awkward

I got a feedback that I can work on improving my debugging and technical skills more

We have some great discussions around these too:

• "How can I root cause bugs faster?"
• "How can I become more independent and better at unblocking myself with tricky technical issues?"

That is, what kind of preparation/ questions should I have ready to make the most of these sessions?

Tactical note: Have a running 1 on 1 doc with each of those mentors and fill it out beforehand, ideally 1 hour+ before. Ping your mentor before the meeting when you do this, letting them know that you have seeded the agenda and you would really appreciate it if they could read it all before the meeting to maximize the quality of their responses.

In terms of what you should ask about, the main resource you're looking for is feedback. Here's a great video on how to get it: This Is How You Get Feedback - Making The Process Smooth

Aside from that, just work backwards from problems to figure out the agenda. For example, you got feedback that you could improve at debugging. A question you could ask your tech lead is: "Are there any meaty open bugs across the team I can take on to boost my debugging skills?"