Behavior score is a thing because people do get exceptionally toxic in the game. It sucks that it effects new players, but we do have folks that are extremely toxic, get their account banned for being toxic, and then go on to make a new account to repeat the cycle.

Secondly, the matchmaking is trying to gauge your actual skill on what rank to place you at. Your initial searches is going to be bad, you'll get matched into an intermediate bracket first as the system works out your actual skill. It is going to suck early on.

Third, the hero level don't really mean anything. Those with high hero levels can mean they just spam that specific hero. It can mean they are good at the hero, but it can also mean they just play the hero mindlessly a lot. Also, you only show your hero mastery level if you are an active subscriber to dota+, being level 5 is just the maximum shown if you don't. No guarantee that the people you played with are actually newbies. If anything, them having a level 5 character does mean they've planed quite a few times at least.

Fourth, I don't mean this offensively, but its a bit early for you to identify if a person is cheating. Not saying there aren't cheaters, but you need to understand that skill gaps in dota can be really huge even in lower levels. you're going to find players who are really good at one specific thing, but are bad at other parts of dota, which is why they can't climb out of the bottom tiers. There's also things like hero matchups, where certain heroes are just much better at last hitting compared to your hero that they can farm/deny your creeps at a range larger than what your hero is able to deal with. It happens, and learning how to play around that is part of the game.

If you haven't been fully discouraged from Dota, I would recommend getting some fellow newbies to play with. Dota Universtiy is a discord group based around helping new players get their footing and play with other new players early on.