Solo games?

If you're looking for a solo ttrpg, check out IronSworn for a semi-nordic low fantasy game that plays great solo. They recently released a sci-fi version called Star forged that's worth a look if you prefer that genre.

You say you're open to anything, but can you clarify if you prefer board/card games or TTRPGs?

There are a ton of free solo TTRPGs all along the spectrum to simple journaling games to full on RPG experiences. Alone Among The Stars, Thousand Year Old Vampire, Starforged, Colostle, and tons more. If you're into that idea, check a few out - there are lots available pay-what-you-want on itch.

As far as boardgames, I love Nemo's War 2nd Edition as a solo game. Many of the Tiny Epic games have solo modes that are pretty fun. Someone else mentioned Friday which is good fun, too.

Not gonna lie, I was today years old when I learned about TTRPGs, or at least the proper name for them. I'll definitely look into these, as they seem very interesting. Thank you for the suggestions!
- Amazing! Welcome to the world (or worlds?)! There are so many different types of games that fit under this umbrella that it can be a little overwhelming.
  
  The games I mentioned in my above comment offer a pretty good cross-section of different types of games built for solo. But there are some I didn't touch on. And there are also plenty of different types of multiplayer games with lots of different themes and mechanics - it's not just D&D anymore.
  
  If you want any more specific recommendations or tips as you explore and get started - or if you just want to share about some of the games you've been playing - definitely start a new post about it in this community. I love chatting about these games!

Some of my current favorites:

When I want something very easy to get into and relaxing: Orchard. Very quick setup, simple gameplay, focused on a single goal
When I want something a bit heavier but still pretty chill: For Northwood, a solo trick-taking game (how can that even work? It does!). Recent game, inexpensive, good replayability, great value.
When I want a nice comfy midweight euro: Newton is a great Italian-style euro that has a very smooth solo. Also online on Yucata.
When I want something heavier but still pleasant: Fields of Arle or A Feast for Odin, some of Uwe's best. Arle is also online on Yucata, Feast is on BGA.

Maquis and Under Falling Skies are my main solo games at the moment. Both are worker placement, but UFS is a dice worker placement game, where you roll a handful of dice and then decide where to spend them. Maquis has a "resistance helping to defeat the Nazi occupation" theme and Under Falling Skies is basically Space Invaders re-imagined as a board game. Both are well done and play in 20 or 30 minutes usually.

Right now there's no game I'd recommend more than Regicide, a game that you can play for free using any standard deck of cards. It works for solo or coop, is surprisingly tactical and challenging and all the rules fit together very nicely.

I learned about it from this Shut Up and Sit Down video, if you want to see someone pitch it better than I could.

I see you've already gotten a lot of great suggestions, but I'll add two more that I haven't seen yet:

Spirit Island: A fantastic game in which you play a nature spirit defending their island from the invading settlers. Great opportunities for tactical decisions, with very little luck involved. All the different spirits you can play are unique, and make each game entirely different.
Sprawlopolis: A Button Shy game, this game consists of 18 cards only (it fits in your pocket). All cards have city blocks, parks, and roads on them. You put the cards down one by one, adjacent or (partially) on top of others, and build your city. Each card has scoring conditions on its back, and every game you pick 3, so you always have different goals for winning. Pretty challenging game! It's quick to play and due to its size you can bring it anywhere with you.

Terraforming Mars is pretty fun, with the base game having a 1 player mode and options for up to 4 players.

There's also a good mobile port which can be really useful for learning all the rules and playing online.

Can't go wrong with the solo games published from Buttonshy. Sprawlopolis and the other -opolis games are fantastic puzzle games. Rove is another puzzle game that is frequently recommended.

I've personally been playing Maquis, it's a worker placement game that has scaling difficulty. It's also nice for limited table space.

There are a couple of tin box games that I consider to be great solo games. I'm not sure how to classify Doom Machine but the goal is to destroy the Doom Machine before it defeats you. You gain dice to add to your pool when you destroy the components of the machine. It's a pretty simple set up that has high replayability. Tin Helm is another easy to set up game that's more of a push your luck. You defeat monsters and delve into a dungeon in order to collect shards to win the game.

As far as TTRPGs that are solo, I'd recommend Apawthecaria. It's a complete package that all the rules you need to play are in the one book. Put simply, you are a woodland critter gathering reagents to help others in need. Cure ailments and possibly help with issues as you do what you can to gain reputation with the guild. It's a bit on the crunchy side. But thanks to that, it's easy for me to wind down when playing it.

Arkham Horror LCG
Lord of the Rings LCG
Gloomhaven / Gloomhaven Jaws of the Lion
Descent, either variant
Legacy of Dragonholt
Tainted Grail
7th Continent

Friday is a very fun solo game.

https://boardgamegeek.com/boardgame/43570/friday

You are Friday and your objective is to keep Robinson Crusoe alive.

It's challenging and got a lot of replayability.

Yes!! I second this suggestion :)

I find the games I'm most willingto solo are games that are quick to setup and have a small table footprint. I love Wingspan and Agricola, and they are fun solo, but I hate the setup/takedown when it's just me.

So my go-to solo games are One Deck Dungeon and Friday. Legacy of Yu looks like something I'd probably enjoy as well, but haven't played it yet.