How are Iranian drones getting their guidance?

They fallback to unjammable guidance, usually INS but possibly TERCOM or AI target discrimination in the modern day. Russia uses sub-$300 hardware like the Nvidia Jetson to enable semi-autonomous loitering strike capability in their Shaheds: https://www.techspot.com/news/108579-russia-field-testing-ne...

GNSS is also cheap, precise and difficult (although possible) to jam. It's probably the preferred guidance on strike drones operating outside the regular theater, where GPS are less denied.

You can use any gps capable phone to provide guidance.

Jamming GPS can be done, but the problem is that the strikes are so sparse that widespread jamming of GPS does more harm to friendlies than good against drones.

GPS is based on a small number of satellites in high orbit, so it's not practical to disable it for a particular region like it is for Starlink and other LEO satellites.