Teen it varys so much by Uni/Type of course. For a huge number (including many russell group) it will be automatic selection depending on whether you hit the predicted grade. For some top uni's who do rigorous selection based on tests/interviews, all that matters to them will be wider interest and intellectual curiosity and ability in the chosen subject. So GCSE s and extra curricular outside chosen subject become less important too. DD2 took all gold D of E, prefect, sports etc stuff off her Cambridge application to make more room for discussion around the chosen subject.
There may be some courses where they will be relevant, including some medicine and US course. I think DC need to either pick uni courses to apply to that match their strengths or do some research and try and get what they need. For the majority though GCSes and extra curricular don't mean a lot.
I also think extra curricular, work experience and things you can talk about at interview will be important for those like DS who may be going into work after A levels and not uni. As I allways say too they are more important for what the DC actually learn and gain from them than the benefit they actually gain on a CV/application