Once you/they decide that certain exams are necessary, you find an exam centre, buy the texbook, grind through the syllabus, do practice papers, and then go in and sit the exam. It's seen as a means to an end.
Studying for the exam usually takes place in home ed over one year rather than spread over two. Although there are exceptions, many home educated children take far fewer exams than children do at school.
Some families choose exam subjects based on what the young person is interested in, others choose subjects they think will be necessary to progress to FE or university. Still others pick subjects for pragmatic reasons where there is a nearby centre to sit the particular exam, or a home ed group is offering shared tuition.
I've got lots of info on my website about home education and exams eg here edyourself.org/articles/examshomeedexp.php
My son is 19 and hasn't done any exams, largely because he's not interested in college or university. We home educate autonomously but "being autonomous" doesn't dictate about exams.
For me, autonomy is more about self-government and questioning why you are doing everything, being driven by intrinsic motivation, rather than "being free to do whatever you feel like."
There's a weekend conference on autonomous home education taking place in Surrey in mid-July if you are interested.