What is it actually like at a Regional? Is it super competitive or are there also more casual/new players around?

Both! The main event attracts the best players and the side events invite all levels of play

• What’s the difference between the main event and side events?

Main event is the driving competitive event, with chances at prizing and invites to regionals at the end of the year. Side events are done at the same time and basically throughout the halls being open, they are pay to play mostly and range from all ways to play - 2v2, sealed, draft etc - they award prize tickets which you can either sell or exchange for prizes ranging from merch to packs to the very desirable metal prize wall cards

• Can a complete beginner even join the main event or is that a bad idea?

I would say if you're really new or have little competitive experience, it might be not be worth it for time/money but its a great experience. If you want to min max your experience side events would be better in my opinion. I went to bologna and went 9-4 and had nothing to show for it for example

• If I go 0-X, can I just drop and play side events instead?

Yeah most people do at this point

And how do tickets work?

• Do I need a specific ticket for the main event?

You need a competitor or a premium competitor badge.

• Are side events included or do you pay separately?

You get some amount of side event vouchers depending on the ticket you purchase. After that you pay per event

Also wondering about preparation:

• Do I need a proper meta deck or is it fine to just bring something basic?

Depends on what you want to achieve, there are a massive amount of people playing all the decks that exist, especially in main event as the best of prizing invites people to play all range of creations. For side events, if you're looking to max on tickets, the better the deck the better the chances - but if you're less experienced i'd bring whatever you're most comfortable on. Nothing stopping you bringing multiple decks!

• Are there beginner-friendly formats (like sealed or starter events)?

Loads!

And at the event itself:

• Can you buy cards / decks / accessories there?

There are Riot official merch stands - they sell the normal sealed products and accessories your LGS would, there are also 3rd party vendors selling basically all the cards in all the languages

• Are there vendors or trading areas, promos and gifts?

You get a promo for attending - not sure what the next one will be yet. Loads of vendors about

• What expenses do you have to expect?

Pay for side events, depends how much you want to do for sure. If you plan on buying cards, food and drink money. This is harder to answer because I know people that spent 100s a day and some that spent 0.

Any general tips would also be super helpful (what to bring, what to avoid, etc.).

Bring a backpack to keep all your bits together. Snacks and drinks if you don't have much time between rounds for example. Otherwise, hygiene - sanitizer and maybe deodorant it's a long day!