SIM card is internal. It records everything until it rewrites over it. Usually about 200 hours of recordings. The service comes from data transmission to the host company’s server where you can access it. If you don’t like that, you can hook your cameras up to a DVR in your closet or attic.
According to ChatGPT:
Nest doorbells do not contain a SIM card for cellular data, nor do they have SD card slots for local storage. They rely on Wi-Fi and cloud storage, although they possess minimal internal memory (about 1 hour) to back up events during Wi-Fi outages. Video storage depends on a Nest Aware subscription: either 30-60 days of event video or up to 10 days of 24/7 history.
No SIM Card/Local Storage: Nest cameras are designed to upload footage directly to the cloud. They cannot use cellular SIM cards.
Offline Storage: If the Wi-Fi drops, the camera can store roughly 1 hour of event footage internally, which uploads to the cloud once connectivity is restored.