I wonder about this too. Can I plug my laptop’s USB-C charger into my phone? Or is that a big nono
I charge by Bluetooth headphones ‘pod’ with my Steam Deck charger and it seems to be ok.
Yes, you can. The charger and the device communicate between one another what they can support, and pick the highest one they both agree on.
E.G. my laptop charger can charge at full speed (100W) for my MacBook, but only at 20W for my iPhone.
That bit is pretty straightforward and transparent to end users (there are a few rare conditions where devices might not agree on the fastest, and have to fall back to a slower one); the issue is more with cables not having sufficient gauge wire, or missing connections that prevent the charger and device from communicating their full functionality.