The beta program is where you get to trial pre-release-sometimes-buggy- software and in order to do it you sign up to the developer program. iOS12 is finished and the beta versions are of 13 now.I am excited to read iOS 13 may not slow down older devices (I have an iPhone 8 & iPad Pro 10.5)...but how do I load the tvOS profile? I haven't seen any instructions yet. Thanks.
I'm not sure how the tv bit got in there but I think that's for tvOS I don't think you use the tvOS for the phone.
Anyway, if you are on 12 and you have recently updated then you already have the last and most stable version of 12 there will ever be and if you don't want to update to 13, all you have to do is go into settings and turn off automatic updates.
It'll stay on 12 until you decide otherwise then.