Genshin Impact questions.
1. Can i play it solo?
2. Can i play it without paying anything?
1. Yeah, the "adventure" is solo play only, which kinda sucks in a way because you can't join a friend to do story missions.
Repeatable content like dungeons and bosses can be done multiplayer, it tends to be faster, not always easier though. A few of the minigames added were multiplayer only, if you really don't want to play with others, you'd miss on some (cash shop money) rewards for those.
2. You can complete the entire game, and there's many hours of game play (just today they added an archipelago for the summer event with hours of exploration - but it seems it's temporary for some reason? You have 40 days to do it if you want to start), even more so with the third region coming next patch; for free.
At no point will the game asks any money, and you certainly won't feel like you need the latest 5* character (the game is too easy in general - but it also means that power level is not too different between chars, though a support char is not a DPS, so it's not just about kicking things down but about making a coherent team). And you'll be able to get the 5*s you want as long as you just play the game and do dailies.
The larger issue is that you'll be limited by your adventure level (it's an "account lvl") to progress through the entire story, which requires you to open chests, do quests, dungeons, bosses, etc.
If you play casually, just doing things here and there and your dailies fairly often, you won't feel it. But if you just want to blast through the game, you might get limited by their stamina system to get enough adventure points later on - it slows down quite a bit at rank 30+. You could spend cash shop money to speed it a bit, but that's mega whale territory.
Just consider that you may need to play a bit every day and not large chunks at a time in the very late game.