So I bought a brushless motor with a fitted ESC. Since I want to power my arduino via a lipo battery, I need to use the BEC, but it‘s an integrated BEC. How do I use that to power my arduino? And how do I wire it?
What specific motor are you using?
The REELY 1:10 offroad
Assuming it has standard servo connector the BEC provides 5V on there between the positive (usually red) and negative (usually black) wires.
Bear in mind that I've personally witnessed two ESCs explode in flames, it might
be safer to have an alternative supply for the Arduino. On most ESCs its just
a 7805 regulator anyway.
Many ESCs have switched mode BECs these days and are much more useful with genuine current ratings of 3A and over.
But I couldn't find any specifications for whatever the OP is using so who knows?