Clear is different, as it's not part of TSA or .gov...you can sign up for it right away at the airport. Just answer a bunch of questions at the kiosk (they use whatever database it is like when you sign up for a credit card/loan/etc so it gives you a list of previous addresses you need to pick from, relative's names, etc), and it gets a few biometrics (eye scan, thumbprints, etc) and decides which one is most identifiable.
So at the security lane, you just go up to the kiosk (it uses thumbprints in my case), the Clear rep verifies that what comes up on the screen matches your boarding pass, and you go past TSA right to the xray.
It was a damn good sales pitch, honestly. I got to EWR and was looking at an hour+ in the regular line, the Clear rep saw the disappointment I had, and politely offered that in 5 minutes I could sign up, get activated immediately, and have a 3-month free trial. 6 minutes later I was at the xray.
It's more $$ than Precheck ($10/mo) but at that time I was flying 2-3 times a month, so who cares. That's less than $2 to skip the line. It's more than airports as well...stadiums, concert venues, etc. For stuff that needs it, you can upload a pic of your vax card and it'll add your status to the verification. (wink)
I guess there's an argument to be made about a 3rd party company having your ID and biometric stuff on file, but I don't care that much.