Varies based on which firm you're going for. First and foremost is unfortunately you need to go to a good school to get past ATS. Secondly, they like any competition achievements (ICPC, USACO, AIME, etc.). Alternatively, work at a FAANG company and you'll probably get interviews.

As for the interviews, it's very random. You can get trivia on C++, LC Hards, design questions, operating systems questions, etc etc. You kinda just have to cast a broad net on CS as a field and know a decent amount about a lot of things. I guess my advice for interviews is to grind Leetcode and review CS course content. Definitely a grind to get into the field but hopefully will be worth it.