Which organizations govern physical layer standards and what's the difference from upper layer standards?

Question

Accepted Answer

Upper-layer protocols are software, mostly governed by the IETF; the physical layer is hardware, governed by a mix of bodies — ISO, EIA/TIA, ITU-T, ANSI, and IEEE.
The split exists because the two halves of the stack are fundamentally different kinds of thing. The upper layers of the TCP/IP model are implemented in software — protocols like TCP, IP, and HTTP — and are largely the domain of one body, the Internet Engineering Task Force (IETF), which publishes them as RFCs.
The physical layer, by contrast, is implemented in hardware — cables, connectors, radios, voltages — so no single body can own it. It is governed by several standards organizations, each contributing in its area:

ISO — International Organization for Standardization
EIA/TIA — Electronic Industries Alliance / Telecommunications Industry Association (e.g. the TIA/EIA-568 cabling standard)
ITU-T — International Telecommunication Union (Telecommunication Standardization Sector)
ANSI — American National Standards Institute
IEEE — Institute of Electrical and Electronics Engineers (e.g. the 802 family for Ethernet and Wi-Fi)

Why it matters: hardware from different vendors only interoperates because they all build to these shared, openly published specifications.
Go deeper:

ANSI/TIA-568 — Wikipedia — the TIA/EIA structured-cabling standard (and source of T568A/B), a concrete example of a hardware standards body.

IEEE 802.3 — Wikipedia — IEEE's Ethernet family covering the physical and data-link layers of wired networks.