In wireless audio the reference design is the chipset. An earbud house picks a system-on-chip, then builds the acoustics, the industrial design, the firmware and the sound tuning around it. The SoC decides almost everything downstream: codec support (SBC, AAC, aptX, LDAC), Bluetooth version, power budget, and how good the noise cancelling can get. The names to know are Bestechnic (BES), Airoha, Jieli (JL) and Actions for the volume tiers, and Qualcomm QCC at the premium end.
The current battleground is noise. ANC (active noise cancelling) quiets the world around you; ENC (environmental noise cancelling) cleans up your voice on calls. Both lean heavily on the SoC and the microphone array, and both are where a good house earns its margin over a generic assembler. Ask any supplier which silicon they run and why, an answer separates a design house from a box-shifter.
Geography is a corridor, not a point. The audio cluster runs across Shenzhen (Bao'an), Dongguan and Huizhou, an hour's drive end to end, sharing acoustic engineers, mould shops and component supply. Houses offer OEM, ODM and white label from catalogues of twenty-plus ready models, carrying FCC, CE and RoHS plus Bluetooth SIG qualification for export.
The apex of this same supply chain is AirPods, assembled by Luxshare in Dongguan and acoustically driven by GoerTek in Weifang, Shandong, both EMS giants, not catalogue houses you can hire. The brands you actually compete with (Anker / Soundcore, Edifier, QCY, 1MORE) sit one layer above the hireable ODMs below.