I'm looking for a USB adapter (for now only to connect my phone, but also headphones in the future) and I was wondering if all of them work with aptX?
Nothing specifies it in the descriptions, so I'm assuming aptX is only a thing on devices like headphones, and the receiving end doesn't matter?
Aptx is a specific feature for the adapter, if you want a USB adapter with aptx you need to get one that specifically has that. Most of the bluetooth dongles you see are mostly for pairing things like keyboards and mice not so much for high quality audio.
The aptX audio codec is used for consumer and automotive wireless audio applications, notably the real-time streaming of lossy stereo audio over the Bluetooth A2DP connection/pairing between a "source" device (such as a smartphone, tablet or laptop) and a "sink" accessory (e.g. a Bluetooth stereo speaker, headset or headphones). The technology must be incorporated in both transmitter and receiver to derive the sonic benefits of aptX audio coding over the default sub-band coding (SBC) mandated by the Bluetooth standard. Products bearing the CSR aptX logo are certified for interoperability with each other."
For music over bluetooth I have a FiiO BTR1K and use that as the DAC and bluetooth adapter to the computer.