Smartphone shows 2 ° C + the laptop shows 7 ° C + for my location. Although the devices are next to each other 1 ° C-2 ° C, I had more often but equal to 5 ° C difference?

Different apps i know. But where do they get their information from?

It depends on which weather station the data is collected.

Can I find out which weather station they use?

I do not think there's only one of those near us that are brought in for the area. For big cities it could be different. But how about using a simple outdoor thermometer?

The hold the data from different providers and in turn from different weather stations. If there's no proper weather station in your area then it will be charged.

I have to take antifreeze measures at low temperatures in the evening that's important…

Buy a good thermometer, only then you have accurate values for your location.