If you're referring to the 'Current or most recent year courses' section under the 'Education' section, you'll want to select trimester, meaning three academic terms, and full-year, meaning you're studying your BTEC for the whole academic year, not just one term.
If you're referring to the Courses & Grades section, first work out if you actually need to input it; out of the 19 US unis I applied to, only two, Stanford and USC, that even used it. But if you do need to input it, this is the exact email I received from my USC admissions counselor about it:
"You can leave Grade 9 blank and for Grade 10 input your external GCSE results.
For Grade 11, you can leave them blank and for Grade 12 input your predicted marks."
For the 12th grade courses section, for 'grading scale', if it's not available, put other. For 'schedule', put other and then under 'please describe', put biennial, meaning a two year course. For 'course level' just put N/A. You'll also want to tick the N/A box for 'credits'.
As part of your application, your school will send a transcript of your GCSE grades and your BTEC and predicted grade in that, so don't worry if the Courses & Grades section on the Common App doesn't contain the phrase 'BTEC'. As I said, most schools won't use the Courses & Grades section, just your transcript, and for the schools that do use the Courses & Grades section, they'll also have your transcript to provide more info.
Just a side note, you may want to have your sixth form/college include info about your BTEC: the fact that it qualifies you to apply to university in the UK, what a level 3 extended diploma is equialent to A-level wise, a brief description about what you study etc. I could be mistaken, but I believe that
@ry7xsfa said that they did a BTEC course but that some US unis were unfamiliar with it and confused by it. I'm sure they'll be able to provide better advice and info.
And great question - a very important answer. No, your Common App essay should not be written like the UCAS personal statement.
The easiest way to understand what the Common App essay is supposed to look like is probably to go on Youtube and search 'reading the essays that got me into' and tons of videos of students reading their Common App essays will come up. I highly recommend you go on a Youtube binge and watch several of them.
I also highly recommend reading this very popular (and for good reason) free online short book, which I personally think offers some of the best advice out there on how to write a really strong Common App essay.
https://www.hackthecollegeessay.com/uploads/1/0/9/5/109505679/hack_the_college_essay_2017.pdfNo, don't send off the Common App this year. The Common App you're currently on is for those applying to start in 2021, not 2022. On August 1st, the Common App will roll over to 2022 entry. Any answers to essay questions will disappear, so make sure any drafts you have are saved somewhere on your computer, though iirc your other answers will remain.
An important thing to know is that the supplemental essays for each universities may change year to year; the ones you are seeing right now on the Common App are the ones for this year's application cycle, not the next. If you want to find out how likely the prompts are to change, you can google X university prompts 2020/2019/2018/2017; if they have been the same for the past few years, there's a good chance it will be the same prompt next year, so you can likely write your first draft, or at least brainstorm ideas, prior to August 1st. And if a certain college's prompts typically change every year, you don't want to be wasting your time writing this year's supplement. NB: some schools will release their prompts in June or July on their websites, so keep your eyes out.
I would also recommend waiting until around October to begin sending your applications off, as between now and then you will have precious time to develop your extracurricular activities, which are really important for admission to US universities, and you'll also need time to write, redraft, and redraft your Common App essay and supplements.
One piece of advice for something you should probably do now: quite a few US unis consider 'demonstrated interest' in their admissions review, so I would highly recommend adding yourself to the mailing list of any college you're at all interested in; you'll also hopefully get some useful information as well. Do you have any schools you're currently interested in?
And don't worry - I'm more than happy to answer any questions you have =)