I've had a ton of fun working with TC39. I feel really lucky. It's been very rewarding to have an impact on the language that I love and to have so many phenomenal engineers and experts to learn from.
In terms of meetings, they are three days, 8 hours a day, with social events and break out sessions outside of meeting hours. Last meeting we had around 50 delegates all in one giant meeting room United Nations style!
For introverts like me, the meetings are incredibly draining, but I think they are also lots of fun. Delegates bring up agenda items related to language proposals they are working on and the committee discusses them. Delegates work to advance proposals through a staged process. And that's pretty much it at a high level.
Outside of meetings is where most of the TC39 work happens. Delegates are always working on their proposals on GitHub, and there is always editor work to be done on the main specification. I keep tabs on most of the proposals and am deeply involved with a few. These proposal repositories are a great way for everyone reading this to influence the future direction of the language as well :)
I spend probably half my time doing TC39 and standards related work, and I'm very thankful Microsoft lets me spend this much time doing it :)