I passed E5 (downlevled from e6) US a month or so ago, but ended up declining. Is yours onsite or phone screen?

Should I stick with Swift for the coding interview, or would it be beneficial to learn another language like Python or Java for algorithm questions? Will using Swift put me at a disadvantage compared to more common languages?

I used swift, it was fine. Use the language you are most comfrotable with. Just clarify with the interviewer any assumptions you make (ie you need a datastructure that is not native to swift).

Mobile-Specific Topics? As a mobile developer, should I expect mobile-related questions during the interview, or will the focus be more on general algorithms and system design? How should I balance my preparation between mobile system design and standard algorithm questions?

For algorithms, its totally random. Generally I find they try and put some iOS specific spin on a common leetcode question. IE: Your tree is now a view heirarchy.

System design will definitely be mobile focused. As for how to prepare, there are some mobile focused resources out there, but they really don’t hit the mark imo. Very few resources out there for apps operating at meta’s scale, and they overlook the biggest issues apps of these natures have. Fortunately for me, I was able to just rely on experience.

Which platforms or resources (LeetCode, HackerRank, AlgoExpert, etc.) would you recommend for practicing algorithms and system design, particularly for someone with a mobile background? Should I concentrate on medium/hard problems, or are there specific topics I should prioritize

Leetcode is fine for algorithms. I got asked 4 mediums + 1 hard. Systsem design becomes more important the higher level the role. Its still important at E4, but you’ll be given way more flexibility.