You are correct that, in the US, at least you are expected to just be able to teach at University. In Europe that isn't assumed and there are actually requirements for teachers.
But the osmosis thing, while true, is a terrible assumption. Most people coming out of a doctoral program have been associating for several years with people just like themselves. Idea driven, hard workers, readers, note takers, explorers, etc. Then they wind up in an undergraduate course, for which they understand the material well, but they don't understand that the students are not all the same. The students in particular are not at all like you or the people you've been working with.
Every student is different. They have different strengths and weaknesses. Many of their weaknesses were caused by poor teaching in the past and they never learned how to learn. Some found it easy early on if not many expectations were put on them and then come up against a wall at University. I've see 3rd year (out of 4) undergraduates who had no idea about how to learn anything from a lecture (in CS). They didn't come prepared to take notes. They didn't come prepared to ask questions. They didn't know how to analyze and compress the information they were given. So they struggled with assignments. I actually had to teach them how to take notes. It isn't hard, actually, and doesn't take a lot of time. Three or four minutes at each end of a lecture.
My students weren't lazy. They just had no clue about study. No one ever took the time to teach them how to do better. It wasn't supposed to be my job to do this, but they and I would fail if I didn't take on the task. Be aware. Don't cut them slack, but show them how they can excel.
But you, the new instructor, weren't likely like that. You think (know) that the students will learn just like you did and that emulating your best professor is just what they need. Nope. Ain't so. That professor worked for you, and people like you. You need to learn about responding to student needs and to look for cues that they are or aren't getting it. Watch their faces. Watch what they do with their hands. Find a way to get questions.
Neither is it true that (in most places in the US) that you will be judged only on research. Some places let a faculty member decide where they want their main focus to be. I worked at places that had three sets of criteria for advancement (teaching, research, service). You needed to be good in all and were expected to excel in at least one. You could define the parameters to some extent, but had to show (in a dossier) that you made contributions to each.
Certain academic positions, in which you advise only graduate students feel a bit different and there the work with students is closely tied to research. That is fine, but you also need to be aware that at the beginning, each student needs help on things other than the topics/skills of the field. But even there, your service to the students will be noted by your peers.
But if you are allowed to teach, then you are likely expected to teach well. It is sad that new faculty in the US, at least, don't get more support for that. But you can, perhaps, find a mentor in your department, known for his/her teaching who can give you hints about both teaching and managing the full range of expectations put on you.
Furthermore, at many (I hope most) places you get to input into the tenure process. My experience is that the candidate writes a dossier detailing all of the things they contribute. Depending on the institution that may be heavier on research or not. But part of the judgement of you will be base on that dossier: Is it appropriate? Did you fulfill your own goals?
Many places also give candidates a review at the half-way point to the tenure decision, giving them feedback on their progress and advice about what they should do differently if anything.
