I had onsite interviews for Google and FB during the last 2 months:

I have a PhD from a US univeristy in the top 10
4 years of work experience in a Fortune 50 Corporation

Preparation:
I have done 300 LC problems 10% easy, 50% medium, 40% hard... I'd say the only problems that I cannot solve are some of the DP ones...

Google:

Phone:
They said that I could skip the phone interview cuz of my background...

Onsite:
3 graph questions and 1 DP question and a behavioural . I did the 3 graph ones and the behavioural well and the DP, I struggled. I received absolutely no feedback of the rounds. It was very interesting that the recruiter refused to give me any feedback about any of the rounds... I have seen people explain that they had lets say 2 good or 2 bad reviews, but I did not receive any feedback. Just a cold reject...

FB:
Phone:
One medium Question, I coded well

Onsite:

I had 2 rounds of coding, one round of ML system design and one round of CV (computer vision) and one round of behavioral. The 2 coding rounds had 2 coding medium level questions and I passed both. The recruiter also said that I passed both the coding rounds. For the CV, I did not do well, the engineer was pretty mean... She asked me how many categories were in the IMAGENET dataset and I did not know how many... She started laughing and mocking me... The recruiter got back to me with a reject because of the ML and CV rounds... Next time, I will choose a Software system design instead... I guess it would have more normal interviewers....

In general, I found the FB recruiter much more helpful. At least, I know which ones I did well and which ones I need to improve... Long story short, it doesnt take just effort to get into FAANG, you should also have some luck...

For ML rounds, I went through:
https://research.fb.com/the-facebook-field-guide-to-machine-learning-video-series/
Also, I went through ed***.io course for ML design,
Also, I read the book Hands on Machine learning and data science with Scikit learn to brush up on algorithms that I have not used....
Finally, I did a couple of interviews in in**.io...

My general ML design review was good, not excellent, but the CV, was pretty tricky... I was given the option to do a System design or the CV; unfortunately, I chose the CV and it did not go well....

All the FB questions were on LC under FB. The Google questions, were all in LC but not necessarily tagged as Google...