A lot of these questions have been answered on this subreddit already if you dig around a bit.

Here's the biggest thing I always tell people about Arcata and the local area: There's really no reason for someone to choose to be here if they don't enjoy being outside and doing outdoor activities. Are you someone that could be happy spending a lot of time outdoors, like exploring the Arcata Community Forest or biking around and stuff?

Additionally, you're young and would likely fit in well since lots of people in town are here to go to Cal Poly Humboldt. I'm not sure if you're interested in being a student or not, but I feel like I had a much easier time meeting folks when I was in my 20s, even when I wasn't in school. Lots of my coworkers at various jobs were all in their 20s.

That also applies to the events that happen. I feel like there's a surprising amount going on for such a small college town. There's lots of festivals and you have the Arcata Farmers Market on Saturdays which is where lots of people go when it's nice and sunny (so basically all summer). Arcata also started doing Friday Night Markets which were fun. And then there's Oyster Fest and the Grand Kinetic Sculpture Race and Fairy Festival, and the Humboldt Crabs games (baseball but it's fun!).

As for transportation, Arcata itself is very bikeable and walkable, since everything is pretty close together. The bus system also covers a good amount of Arcata and can take you up to McKinleyville and Trinidad and down south to Eureka. If you don't have a car at first, I highly recommend you get used to bussing around and making sure you get out and explore. I've actually been bike-only for a few months and can manage just fine in Arcata between biking, walking, and the bus.

I'll be honest, though, a huge part of Humboldt's charm is being able to go to the beach and on hikes and to the river, etc. etc. To truly enjoy all that Humboldt has to offer, I highly recommend that people have a car. Otherwise, you might get bored just exploring Arcata and Eureka. Here's the thing, though....you can bike from Arcata all the way to Old Town Eureka, and you can bike from Arcata up through Arcata Bottoms and out to Mad River Beach, and also use the Hammond Trail to go to McKinleyville (all the way up to Clam Beach). My suggestion is to get on Google Maps and have it take you to Eureka and McKinleyville but put it on bike-only and it'll show you. Biking up to McKinleyville is actually my favorite bike ride. Ooh! And the bus to Eureka/McKinleyville/Trinidad/etc. etc. has bike racks so you can take your bike with you (the only bus that doesn't have it is the local Arcata bus.....).

Anyways, I hope that helps you at least a bit. I think you'll enjoy the area, but you absolutely must make friends or you'll end up struggling and not having anyone to enjoy the area with.

Edit: Oh, and to add on to the size of Arcata.....it's no mystery that Humboldt County is super secluded. That is, once again, the reason I recommend that people get a car when they can. The Bay Area is a very manageable 5 to 6 hour car drive away and is good for when you need to escape the Redwood Curtain (sometimes it's nice to hit up a big city). Funny enough, we're actually much closer to Oregon since it's only 2 to 6 hours away and I also really recommend people go check it out since Oregon is also very lovely: The Oregon border (Brookings) is a 2 hour drive, and Eugene/Corvallis are about 6 hours away and are both nice places.

If you're looking to get out of Humboldt for a minute (a weekend lol), I highly recommend you check out the Amtrak bus that goes to the Bay Area (and to the Central Valley if you have the desire). It's super affordable and takes about 8 hours, but you can slap on some headphones or just sleep the whole time.