Overall what Google looks for in Algo interviews:

Working optimal code for the main question and follow ups if needed. I had to write code for all the follow ups as well. Clean code, testable code, coves edge cases. Keep explaining your thoughts while writing code. It is difficult but important to convey your thinking process.

Round 1: Stack based problem. Think of evaluating some equation and giving a boolean result.

Round 2: Circular queue buffer based problem. Logging/ Rate limiter/ Thread safe solution

Round 3: Strings. Anagrams/ Palindromic

Round 4: System design. (Mobile app based)
Solution is expected to be modular, extensible, scalable (in respect to number of devs working on this), performant, storage, battery aware.

Round 5:
Behavorial interview
Leadership, team work, collaboration, culture.

I got additional follow up rounds after hiring committee review downlevelled me to L4. (Clearing additional rounds will get L5)

Additional Round 1:
System design: Same as above

Additional Round 2:
Problem: Graph traversal. If you have solved DFS problems before, this should be easily solvable.

Finally, L5 was offered. Compensation post: https://leetcode.com/discuss/compensation/1292524/google-l5-bangalore/987354

Preparation Strategy:
It was seriously not that different from what everyone is doing. Solve medium problems in all the patterns to get comfortable with basics and finding patterns among questions. Since getting to a solution "fast" is a key, it is very important to be able to identify which data structures and algo solves which kind of problems. (Like, if there is a constraint in problem space related to "choosing" items/people, it could be solved using MinHeap/MaxHeap structure)

May be one thing I did differently could be, I took long time for onsite preparation. Roughly, 1 year between phone and onsites. Since I had a lot of time, I was not preparing day and night. I have a 2 year old kid at home so I preferred to spend my free time with her rather than solving DS problems :) Google was fortunately very helpful in this case. They gave as much time as I needed. So, I took my time and only worked on weekends doing contests (Really really helpful in simulating time constrained/stressful environment) and solving mostly medium problems.

Finally, I was extremely lucky to get almost medium problems in all my interview rounds. We all have to accept that luck plays a very big role in interviews but it is only that our prepartion decides the amount of luck factor. The more you are prepared, the less lucky stars you need in your interviews :)