Alan, this is the basic, timed entry ticket you were likely looking at:
https://www.coopculture.it/en/products/ticket-colosseum-roman-forum-palatine_24h/
That page lists both what it does and does not cover. Essentially, it does not include any sort of guided tour, access to the underground or arena (which is the floor which partially covers the underground) or the S.U.P.E.R sites within the Palatine/Forum section.
People choose tours for different reasons, such as their educational value and/or access to what they're not allowed to visit on their own. For instance, you can't access the underground of the colosseum without a tour or being accompanied by an attendant; coopculture offers tickets for both options. A tour which includes the underground? Probably the most in-demand ticket for any Roman attraction, and the most difficult to obtain these days.
Coopculture does not offer a comprehensive tour of the Colosseum, Forum and Palatine together but there are independent companies that do. They usually run 3-4 hours, and although they either don't include the underground of the Colosseum or note that they can't promise access to it, these are good for doing all 3 efficiently, and trying to make sense of the Palatine and Forum. The forum, in particular, can be a complex site to figure out what you're looking at without either a human, printed or audio guide. If interested, give a shout and people here who've done one of the guided tours can make recommendations for who to book with.
If your main interest is the Colosseum and you REALLY want to get into the underground, you might look at the night tours? Doesn't look like tickets are on sale yet for your dates but my hunch is that when they become available, they'll be easier to land than the 34-euro "Wonders of the Colosseum" coopculture daytime tour which includes guided narration; THAT is the hot ticket that's so difficult to get hands on. Do understand the night tour does not include access to the Forum and Palatine, however, like most other Colosseum tickets do.
https://www.coopculture.it/en/products/colosseum-by-night-2022-colosseum-underground/
If you just want to SEE the underground and don't care about guided narration, there is this option: they provide someone to accompany you but that attendant just walks you through. This is still probably a pretty difficult ticket to land but you could try.
https://www.coopculture.it/en/products/full-experience-ticket-with-access-to-the-arena-and-underground-of-the-colosseum/
Did any of this help or just further confuse the issue? Trust me, you aren't the first, and won't be the last to be a bit baffled by the various options! :O)
