I have seen on the web that the tour costs 12$, but I don't want to do a guided tour, just enter a few minutes to see inside, and if it is possible to go to the 7th floor, but it is not clear if it can be done or not without taking part of a tour

Yes, you can do a free self guided tour and access the 7th floor without going on the tour

Touring admission is $12 for adults, Monday through Saturday. It is not possible to enter the cathedral to look around, without going through the tourist admission desk. When entering the Cathedral on any day to attend a worship service, there is no admission fee. Also, once the main Sunday morning services are complete, the building is open to self-guided touring (sometimes tour guides are available) for free.

Give them a call and find out.