How are level 3 extended diplomas and BTECs assessed?

As others have said it is dependent on course DD did performing arts so she was assessed each term in her dance styles and singing. She then had skills section which she choose flexibility so she kept a vlog of gradually improving her flexibility.
There were written assignments on plays, there was a devised acting project where they were assessed of a variety of criteria from organisational skills to the acting.

My son has just passed his btec level 3 in mechanical engineering and manufacturing. It was a combination of practical work and a lot of a level maths

It's very course and college dependent

For us, DD started considerably ahead of what the course actually requires and qualifications at higher level (long story but basically no options at 16 that were in line with prior attainment and universities were reluctant to take until 17 due to hands on nature of course and having an U18 on campus with all the added safeguarding faff)

Even with it being the case that the work was all stuff she's already done at higher level and nothing new is being studied, the format for these courses is very heavy on 'busy' work and so on amount of time out of college.

You can't just say 'yep I know x, y and z'. Everything has to have detailed proposals, descriptions of what you are doing, log books and then reflective appraisal of what you did, what you did well, what you would do better, SMART targets etc.

And those parts take AGES to document and write.

It's not like an A level where, if you were significantly ahead, you could pretty much coast for 2 years and then deliver it all in the exams.

There's also a significant self study block. In Y1, we had weekly 90 minute videos that had to be watched and then tasks completed to prove you'd watched and done them. Top tip... watch the videos on triple speed.

Be very aware of how the mark schemes work, whether your college allows resits and when, and don't think you can leave things till the last minute if you are on BTEC or RSL.

I know a few of DD's friends are devastated to discover that having had a very laid back first year, they now cannot achieve top grade even if they get 100% on every assignment in Year 2.

UAL is different - there everything depends on final project.

There is a very comprehensive explanation of scoring by @OhCrumbsWhereNow on page 8 of this thread: Page 8 | BTEC/T level/other vocational 25/26 - part 2 | Mumsnet

Synoptic assessments. I had trouble grasping them but this was my conclusion (which may or may not be right).

Synoptic assessments assess across a set of modules all together.

So my DD did animal care. Had she done the Level 2 theory course that had synoptic assessments she would have had to eg

Go into a room. Select an appropriate enclosure for eg a guinea pig (accommodation, health and welfare). Set it up appropriately, choose food. Show how to safely transport (transport) the guinea pig from elsewhere to the enclosure. All the while giving a running commentary on what choices she was making and why in as much detail as possible .

eg she would have to cover how much space they need, light, away from other animals, what bedding, shelter, water and food requirements, carry cases and their handling, house in groups all with the whys as well.

We chose for her not to do the theory course because of this as without being prompted she wouldn't have been able to remember to say all the details, even if she knew them. And it would have been way too stressful.

He was in college three days a week if that’s relevant to the question.

I wonder if it depends on the subject. My DS did an extended Diploma in fisheries and aquatic science. There was a lot of practical assessment as you’d expect for that kind of course. And then several written assessments over the year, including ‘synoptic’ assessments (I still don’t really understand what these are). I’m not sure there were any exams at all, although some of the written assessments were done under exam conditions (but with a few weeks beforehand to research and prepare).

If you know the awarding bodies, the assessment info should be on their websites.

BTECs.

There are a number of modules to be done. Some will be compulsory, and then the college picks from optional to make up the total number of units required for the size of the qualification.

Within each module there will be a number of assessments covering all the Pass, Merit and Distinction criteria listed for the module.

To get a Distinction for a module you must meet all the Distinction, Merit and Pass criteria.
To get a Merit you must meet all the Merit and Pass cirteria.
You have to at least Pass and will get resubmit until you get all the Pass criteria.
Note missing just one criteria at a level means you don't get that level.

The overall grade for the BTEC is made by adding all the module scores and seeing where it lands you. iirc roughly you need about 2/3rds at the level to get that level overall. eg MMMMPP would lead to a M.

Extended Diplomas (equiv 3 A levels) the highest you can get is Dist-Star Dist-Star Dist Star and then it goes down (in the same way as combined science at GCSE) so you could end up with eg DMM or MMP for example, but not DPP.

There may be exams you have to Pass too or exams that go to the overall grade.

BTEC Certificate is equiv of 1 A level so you end up with just one grade.

You have to hit the ground running as missing a Merit criteria early in a module potentially caps that module at a Pass (unless resubmissions are allowed which seems to vary)

My son is doing a business extended diploma. 2.5 days at college freeing up some time to work. Mainly assessment based, but he does have some exams after Christmas.