Can generating answers for behavioral questions where I lack experience bite me back?

I would not manufacture stories. If you feel like you don't have a good story, take something which you experienced, either personally or within a team, and talk about what you would have done.

This way you can still share more about the context and constraints, but you have a bit more freedom to talk in hypotheticals.

Okay, "generating" is a bit of a vague term, so just in case, I'm going to say that you should never create completely false stories for behavioral interviews. There's so many ways this can come back to bite you:

• Instant rejection - Interviewers, especially at FAANG, are perceptive as that's literally their job. The behavioral round (codenamed "Jedi" for a reason) is usually given by a very senior engineer (Staff+) or an engineering manager. These are people with excellent social intuition.
• Blacklisting - If a candidate is blatantly making stuff up, they can be barred from interviewing at the company ever again.
• Incorrect leveling and PIP - Let's say you trick the behavioral round and convince the interviewer that you're more senior than you actually are with grand stories full of great experience, getting an IC5 or IC6 offer. Once you start at the company, you will actually need to function at that level. Given the atmosphere of Meta nowadays (look at the 5% "low performer" cut), it's very unlikely you'll make it past your first performance review.

If by "generating" you meant taking an existing story and embellishing it a little, that's totally fine. Behavioral interviews are all about making yourself look good. But do not fabricate entire stories/answers from scratch.

I highly recommend going through our entire behavioral interview course: Master The Behavioral Interview As A Software Engineer

These 2 lessons are particularly relevant:

• https://www.jointaro.com/course/master-the-behavioral-interview-as-a-software-engineer/its-a-conversation/
• https://www.jointaro.com/course/master-the-behavioral-interview-as-a-software-engineer/when-you-dont-have-an-answer/

Will the interview notes/feedback from behavioral round (first round) be available to all subsequent interviewers?

At Meta, every interviewer at least shares the questions they asked with all the subsequent interviewers, but they usually don't share detailed notes. However, it's still possible, especially for more senior candidates.

and will they cross question me about it in the following rounds?

Probably not, but it's possible.

Eg - for driving efficiency if I say I improved the bottlenecks in an ETL pipeline, will the interviewers in the next round deepdive into this? Or If I say I trained a LSTM variant, will they grill me in the analytics round about it?

Anything you say in an interview opens you up to getting grilled about it, especially at FAANG. With subsequent rounds, it's unlikely as asking new questions generates more signal than rehashing prior ones, but the current round obviously will.