With the [help of HN](https://news.ycombinator.com/item?id=41486352) I realized that all GPS tracking devices require a sim card or some form of monthly subscription. You need to pay to send the GPS data to the satellite.
However, a user recommended using an old phone, and I believe even without a sim card, a phone will send GPS data.
There is clearly a whole in my understanding somewhere. Can anyone enlighten me on 1) a GPS tracker that does not require monthly subscription, or 2) explain to me why phones can do it?
No, the device is only receiving the position data from satellites. It then sends the position via the network (internet) to you.
The phone will need network (internet) connection so you require a SIM card. GPS tracking devices usually use special data plans where the providers only allows a couple of megabyte per day, and thus cheaper than any phone plans.
https://www.eiotclub.com/products/gps-tracker-data-plans is $2.20 USD/month.
https://lightbug.io/product/lb-dev-ze2/ for example is 2-3.50 Euro/month if sending every 30 minutes is enough. The company probably pays less than 1 Euro for their data plan before they resell it to end users.