Junior faculty position - course development
I think this is a false dichotomy. In the US, you are generally expected to be able to teach a course your first semester. You shouldn't need to have the course fully prepared in order to start teaching it, though. You can prepare as you go.
Also, many departments in STEM fields back off on their standard teaching load for junior faculty during the first year to give you some time to prepare your first courses, so you might get a 1-1 your first year instead of the usual 2-1 (or whatever). This kind of thing varies quite a lot from department to department, but there is usually some sort of consideration given to let you ramp up some classes.
A position starting in August almost certainly expects you to immediately teach courses for the academic Fall Term. As a junior/first-year instructor, the courses and their general expected learning outcomes will typically be dictated to you, and you will likely be expected to use whatever textbook and resources were indicated while students registered for classes during the Spring and Summer.*
In order to make this expectation reasonable, a few concessions are made. Normally instructors are expected to handle academic advising for a certain number of the students in their subject area, but it's common to waive this requirement for first year instructors. This allows new instructors to devote more office time to preparing lesson plans and the like, and helps avoid advising mistakes from instructors who are not yet familiar with the degree plans in their area. You may also have one course fewer on your load than is normally expected, or you may be granted an extension on when your full syllabi must be posted. Finally, you may be given access to materials from prior or concurrent sections of the same course as taught by other instructors.
But, when it comes down to it, that first year you'll likely still find yourself writing lesson plans the week and day before delivering them. In the current climate, be grateful for a tenure-track position at all :/
*Note that this isn't a matter of infringing your academic freedom; it's just that there won't be time for you to start from scratch, and you should still expect full academic freedom as you get your legs under you in the new position.
When you apply to most positions (except at the very highest tier), you are asked to provide a teaching portfolio. This could include courses that you've taught, syllabuses, teaching evaluations, a statement of your teaching philosophy, etc.
When you give a job talk, you may also be asked to give a guest lecture in a class or before undergraduates as a means of assessing your capacity as a teacher.
The assumption here is that you've already been teaching or been a teacher's assistant -- and can jump immediately into full-time teaching from Day 1.
The university might do some things to make it easier -- a course release or perhaps allow you teach a course where the syllabus is fixed. Or perhaps a high-level seminar where they only expect 3-4 seniors or grad students. But they don't have to and many PhDs have been thrown into 3:3:2 teaching loads with none of that.
Now, everyone knows that the first several years of teaching are difficult. So don't worry if your courses aren't as fully baked as they should be or if you have some negative reviews. Tenure and promotion committees are interested in positive and upward trajectories -- so at least you'll have the ability to show this!