Having been to a grammar school I am aware of them, but equally the concept of them and distribution of them around the country has changed so much (I'm 54) since I was a student, I can empathise with the OP asking the question.
My DS, now late 20s, ended up at the same primary that I went to, and it wasn't until his final parents evening that I was made aware that the admissions process had changed with regards to grammar selection.
In my day, everyone did the test. My primary was eclectic, a good mix of ethnicity and "class" considering we are South Coast and overall regarded as affluent. The catchment was slightly less affluent though because the well off kids went to private schools. But I digress.
None of us even knew we were taking the grammar exam. The subject of which secondary school we were going to was very much an abstract concept and we just waited to be told. I and four others from my school got in.
Fast forward to my son - at the final parents evening, his form teacher breezily said she didn't think there was any point in putting him forward for the exam. When I boggled - bearing in mind I had been an engaged parent in all areas otherwise - teacher went on to explain the new admission policy and that most of those taking the exam had been having extra tutoring for 3 or 4 years.....
After discussions with my son, he wanted to have a go as two of his closest friends were likely to go to grammar after years of tutoring apparently.... When I mentioned all of this to the parents of said friends they looked at me as though I had two heads and should have "just known". I felt like the shittest parent in the world.
Anyway, long story short, he didn't get in, didn't care, went to what became a local Academy and was unsuccessful academically despite being bright.
But actually now he's hard working in areas that suit him and doing alright - I don't think grammar would have helped and over the years, I have come to judge success by many criteria, not just academic achievements.
My own experience of grammar was that because I came from a low wage and mildly dysfunctional background, while I did ok up to O-levels, I wasn't emotionally equipped for discovering that I couldn't afford to keep up with the better off kids, plus as an only child my parents were quite strict, so socialising was regarded as high risk and I predictably went off the rails in college for a while.
Not entirely sure what point I'm trying to make - sorry.
Other than the original premise of grammar schools has apparently been corrupted and many bright kids may well miss the opportunities that the system was supposed to provide.