GCSEs are the main exams taken at 16 but they are not the only exams / qualifications.
GCSE passed at grade A*-C or 9-4 depending where you are in England, Wales and NI are 'level 2' qualifications, lower grades are 'level 1' qualifications.
Level 3 can be A Levels or a certificate / diploma often called BTECs but other exam boards have their own. You can also take IB, depending if you have a local provider.
Scotland has a different education system.
Pros of the system are that the school choses the exam board and all children taking that GCSE sit the same paper wherever they are.
So teachers cannot play favourites or mark down students they don't like.
Everyone understands the system, '5 GCSEs or equivalent with English and maths' is regarded very much like a HS diploma and counts as such if you apply for a US college.
Cons of the system - well it favours the students who are good at exams and there is little incentive to work all year if you are that kind of student.
I'm old and we did O Levels and CSEs which were amalgamated into GCSE. What often happened under that system was:
a) academic students being entered for one or two exams a year early giving more time to concentrate on the other subjects
b) you could sit both a CSE and an O level in the same subject, very common if you were borderline on a pass.
c) you were only entered for the exams your teachers though you could pass.
At present schools are constrained by 'progress 8' which means the school's 'rating' depends on the 8 GCSEs the student sits in their final year. So academic students can be twiddling their thumbs for 6 months and less able have to sit exams they may not be ready for. Or have no interest in. The government also weights subjects differently so there is bias towards STEM.
Things have also changed uni wise. When I left school in the 1980s nursing was not a degree profession, you worked and trained at your local hospital and entry was the aforementioned 5 O levels including English and maths.. Now you have to take a degree.
FE colleges used to run 'consolidation' courses which were basically resits in a year, these have disappeared so outside school it is difficult to do a reset of anything other than English and maths.
You can actually leave school at 14 and transfer to an FE college, although it is rare, it does happen. Personally I think it should be used more, schools in England (I can't say for the rest of the country) don't have the facilities to teach things like mechanics or catering.