I had an internship with Amazon last summer and was lucky enough to have the return internship offer for this summer. I am quite nervous because my tech stack skills are so weak. I am just good at technical interview because I do competitive programming and leetcode a lot, thats why I land my internship offer with Amz.
I forgot almost everything from my last internship. I have to admit that I was a very bad intern, not to say the worst intern. My mentor and the senior engineer of my team did the most of my last project so I got the return offer.
Please suggest me any sources to improve my tech stack skills, projects, open sources, ... so I can perform well at this summer. My team last year used Kotlin for backend codebase.
First of all, give yourself some credit for converting the last internship. Although it's important to be aware of areas we can improve upon, for your case, I would put way more emphasis on things you did well and the signals you showed your seniors at Amazon that led to the return internship. Form a couple hypotheses, then seek some feedback on whether others view these things as impactful/helpful.
Honestly, don't worry about your mentor and senior doing most of the heavy lifting since interns are not in the hot seat to generate a ton of output. I would think back to what your mentors / seniors did that was well received by the team / leadership. If you genuinely feel what they did was great work, tell them and get some input from them on how you can reproduce that success / contribute more.
Overall, work backwards from the outcome you want, which is a FTE offer by the end of this internship. Figure out the scoring rubric if you can and get feedback along the way on where you are progress well vs. not. It's similar to using office hours at school to get early feedback on assignments before you have to submit for the final grade.
I'd really recommend keeping in regular touch with the people who oversee you and make sure you're effectively contributing to the team. Like already mentioned, interns aren't expected to do much heavy lifting, but do even better than you did last year and see how it comes out. Even better if you can use the languages you're using more on a day to day basis and see where that lands you.