Basically, the EBacc measure - in league tables and school data - was introduced to encourage schools to encourage as many of their pupils as possible to take what could be regarded as 'a decent basic set of GCSEs', especially as the accountability measures at the time could be (and were) 'played' by schools doing 'GCSE equivalent' qualifications that looked good in league tables but weren't necessarily leading to a balanced set of qualifications at 16.
Many pupils in many schools were doing - and continue to do - those subjects as a matter of course, but it has perhaps nudged more pupils and more schools towards e.g,. studying a language to GCSE.
The only 'glitch' in it is that only Geography and History count in the 'humanities' pot - schools where RE / Philosophy & Ethics is very well taught sometimes have a lower-that-expected EBacc score because pupils may take full course RE or Philosophy & Ethics as an 'essay / humanities' subject in preference to History / Geography.
In my DC's school, options are very wide, in 5 free option blocks, but students are 'strongly encouraged' to select an EBacc range of subjects (the blocks are weighted that way, with 1 block being almost entirely languages). Schools 'selecting pupils for EBAcc' presumably have different option blocks available to students they believe to be capable of achieving the EBacc vs other students, perhaps because of timetabling or staffing constraints in some subjects.