We are traveling from Paris to Florence through Switzerland. We booked the TGV train from Paris to Basel separately and are now booking the SBB trains (including the FRECCIAROSSA) from Basel SBB to Firenze S.M.N. What is the difference between SBB Train’s first class and executive class on a trip from Basel SBB to Florence and is it worth it?
I do not see any Frecciarosso trains from Basel, so I assume you are referring to the EC train from Basel SBB to Milano Central connecting to a Frecciarosso to Florence. Executive class is extra spacious. Business class would be closer to 1st class on Swiss trains
The man in seat 61 describes the different train classes and includes pictures:
Frecciarossas trains:
https://www.seat61.com/trains-and-routes/frecciarossa.htm
Swiss trains:
https://www.seat61.com/international-trains/trains-from-Switzerland.htm#Switzerland-Italy
The Frecciarossa is not an SBB train. It is a Trenitalia train. But SBB can book it for you. Freccias indeed have four classes, but that is an Italian thing.
The Basel to Milano train is operated by SBB till Domodossola, and by Trenitalia beyond that. You can end up either on a SBB or on a Trenitalia owned trainset for that part. On that train there is only first and second class.