It seems that most of the seating is outside. How does that work during now when it is cold outside? Is there indoor seating available? Do you need to request it when reserving a table?

Cent'Anni has indoor seating. I'd recommend reservations during prime time dinner hours; especially late week and weekends. Doesn't hurt to mention that indoor seating requested when making reservation. Not sure if it's needed though.

They have extremely effective space heaters in the back patio area as well

Yes. There is indoor seating. They're not going to sit people outside in the cold.