I was with Sunrise for umpteen years with the Google Pixel 5, 6 and now also 7.
With the 7, I was with Sunrise for about 1 month and then I switched to a third-party provider, which also uses the Sunrise network.

Yes, the Google Pixel 7 states that it does/cannot do 5G on the Sunrise network.
No, as far as I know you don't have to do anything special to use Sunrise's 5G network (sign up or anything). But you may have certain data and speed limits depending on your subscription (even if you are on the 5G network).

From my point of view, there is no reason to believe that 5G in the Sunrise network should not work with a Google Pixel 7.

With Sunrise, you have to report that you have a 5G-capable mobile phone and then 5G will be switched on ...

Unfortunately I can't say, I'm with Swisscom and 5G is ok.