Sometimes the hosts of the actual podcast read an ad using their own voices.

Yes, though some of those are dynamically inserted as well.

I sometimes watch years old podcast episodes, and it'll insert a pre-recorded current day ad where the host is doing it. And when I listen to the episode months later, the ads change again sometimes, but it is still the host doing it.

Also, maybe it's just the podcasts I listen to, but all podcasts I listen to seems to always be the host doing the ad.