Here is a good explanation of types of and treatments for termites.
We have the same questions you asked. Hard for me to understand how termites can get to roof rafters without some evidence of their journey.
I know the possibilities however we lived in GA (concrete stucco/wood frame/on slab) for 35+ years without any termite damage. And we paid for annual termite inspection the whole time.
If you look at TV new home construction, you can see how all inside wood is sprayed (about waist high) with a termite preventative that is dyed blueish-green. The dye is there so building inspectors know it has been applied.
We are surprised how much more expensive termite "protection" is here in TV and are still undecided about it's real value.
Don