How to obtain email address of interviewer?

You should ask the recruiter, so the recruiter can give that interviewer a heads up and your message wouldn't come across as weird. You don't want to send an email to the interviewer and have their immediate thought be "How did this person get my email?".

In general, be careful about sending emails to interviewers after the interview as the power dynamic is obviously lopsided. Their reaction will most likely be "This person is just trying to get me to pass them."

I would only send this email if the interviewer was genuinely incredible and you two connected extremely well. Otherwise it will feel out of place.

At Meta, there were frequent complaints internally about employees who got unwanted messages from candidates they interviewed. If the information is not directly presented to you as the candidate, your sleuthing attempts to guess the email may backfire.

If you're doing this for a cold outreach, this oddly feels more acceptable to me, but this method for landing a job has a low hit rate. Referrals are a better bet (see the masterclass on networking here).