Right now, there aren't any US shows that accept unsolicited scripts. In the past, some shows in the Star Trek franchise were open to submissions.
Scripts for fictional TV shows are written by either staff writers or freelancers. Staff writers get hired through an arduous process commonly known as "staffing season."
I guess the last writers' strike changed this, but the old way was so much better.
When I wrote for TV (no sales, but I did get attention, spend
that, I say) I just made phone calls to them, ready to pitch for their show. If they liked the pitch, they'd send me a bible and a few samples. I wrote the scripts, then they'd accept them, look them over, and (in my case), reject them.
One show was just great to write spec for. I had one script shot down by the bean-counters because two scenes were too expensive. The producer loved the tale, though, and suggested it be written in novel form. (This came from the Script Consultant, which I suppose now is the show runner.) Two more were still in the running when the show canceled, and I had the green light on a couple more.
It was nice hearing from them, getting feedback from them. I never got 'liner notes', probably never got far enough, but the pitch 'meetings' were fun for me. I didn't feel overly nervous, and the conference calls had a bunch of people on the other end. I don't recall a single negative statement. Sure, there was the 'I don't think that one is for us, what's next' and off I'd go on the next pitch.
There weren't any guarantees, but at least I knew I was writing what they wanted.
I don't think this new way of getting fresh ideas into existing TV series has worked so well.
I sure hope it goes back to the old way. It's fun, there is communication, and everyone is working to see a good show stay on the air.