Question
do they die off in the winter frost and not come back in the falling summer?

Answer
Snapdragons are hardy annuals and will survive a mild frost. However, they do perish once there is a hard freeze. The great thing about snapdragons is that they reseed themselves. If you deadhead them throughout the growing season, in other words, prune the tops once the flower drops, the seed pods left on the ground will go dormant in the winter and germinate in the spring.

I hope this answered your question. Please write again if I can ever provide assistance.

Regards,

Mike