Hey guys,
I really hope this will help someone, though please do not expect me to reveal any details under NDA.

I started my interview journey with FAANGs 2 years ago after a decade of working for mediocre company. I got rejected by Google during phone screen after solving 20 LC problems, then rejected by Facebook after solving 350 LC problems and then got hired by Amazon on L4 (SDE2) position after solving 450 LC problems. This was my first step to my dream, but I stopped leetcoding for a year after 550 LC problems mark. Then Facebook recruiter reached out to me and I allocated 2 months of preparation for Google and Facebook for L5. I solved around 100 LC problems extra before my recent interviews. There was another post by the guy who got rejected after 950 LC problems solved. I must admit that with that high number of solved problems it's still a lottery. I believe that I got highly unlucky with Google twice in a row, my interviews were conducted just poorly. Like really, I still believe I dodged the bullet and I do not want to make any attempt with Google in next couple of years. Done with them.

Google Interview

1. I skipped Phone Screening. Somehow recruiter decided that I'm competent enough to go to virtual onsite directly (thought I failed phone screening last time).
2. During my virtual onsite I got very doable LC medium problems. I got stuck with one problem and solved it by the end of interview only because I started with unsuccessfully long approach. I would usually solve such problem in 20 mins.
3. My system design went okay. But it was conducted in a very bad way. Interviewer looked like it was his domain of expertise. It's nothing like a typical distributed system design, but more like just a design of a single component that we usually use as a building block in a distributed system. My interviewer wasn't prepared and decided to come up with the question right on the spot. :-(

I was rejected and then ghosted by Google recruiter. Google really droppped the ball in my case second time. I had a bad phone screen experience first time I interviewed with them. So it feels like that Google is not the Google we knew right now. Especially, after hiring so many people for Google Cloud. ;-)

Facebook Interview
Unlike Google, both of Facebook's onsite interviews were conducted perfectly! Good job Facebook!

1. I solved 2 medium problems on phone screen. And during virtual onsite I solved 2 problems per round as well.
2. I have to emphasize that a good portion of problems were very similar to those that are tagged here, so LC premium is highly recommended. There were some interesting twists to some problems, but you have to be fast, like lightning fast if you want to impress. Note that the solution wasn't always the most optimal in my case. If I saw that I cannot code the most optimal solution fast enough, I just agreed with interviewer to code obvious solution fast and then talk about optimization.
3. System design went pretty well in my case. My experience and some popular github repository by FB tech lead was helpful to plan the preparation. There is one interesting YouTube channel by our fellow L6 Amazonian. If LC doesn't ban URLs here, I can try to post it later. (Search for "System Design Interview" channel by Mikhail).

How did I prepare on Leetcode?
When I failed on my first Facebook onsite I tried to solve as many tagged problems as possible and then walked through solutions 2 days before the interview. That didn't help me with some problems, so I decided to change my tactic. Recruiter told me that I should not expect DP problems and that I need to focus on solving 2 LC medium problems in 40 mins. That's exactly what I did.
Every day before starting my work day at Amazon (my work schedule was pretty intense in my case :-D) I allocated at least 1 hour to solve Mocks on Leetcode. I solved all Google, Facebook and Amazon interview. I think this was the road to success. After 2 months of preparation I recovered all my previous skills of problem solving and could complete 2-3 problems in 30-40 mins. Coding under the pressure that's exactly what you need! Sometimes I was able to code solution during last 2 mins of the timer. When I saw the timer counting down last minutes, my heart was pounding like crazy! This helped me to control my emotions a lot. I was in the same emotional situation in Google and Facebook interview when I struggled a little.

Beware, that some questions in those mock interviews are total BS. For example, I have no clue whose idea was to put the problem in Amazon mock deck that can be solved only by applying Tarjan algorith. This is freaking crazy, Amazon would never ask that (unless there is something truly wrong with the interviewer).

I accepted Facebook offer in the end as it's a next level for me and I'm really excited about the opportunity. I hope you can do that too guys!
Good luck! :)