HDMI was invented for HD TV sets.
When they needed to stretch it to higher resolution Monitors, that what they did -- stretch it to as high as it could go without a major re-working. That is what in included in early M1 Apple-silicon Macs, and it is HDMI 2.0. it can do data rates about 3/4 of a USB-C bus top speed, or about 14.4 G bits/sec.
The HDMI standards setters did not anticipate that display technology would eventually include Hi Dynamic Range (HDR) (more colors) or higher resolutions. To go any higher, they needed to make a technology change. They stuck with the old connector, and re-arranged the signals and changed the way data were transmitted. that meant the HDMI 2.1 interface could run at 42 G bits/sec, but unfortunately, that is faster than a Thunderbolt Bus.
in Apple-silicon Macs, M2 Pro and later included DIRECT HDMI port that is also faster than a Thunderbolt Bus, and can run at the required 42 G Bits/sec speeds required by HDMI 2.1. Those limits are 4K at 240 HZ, or if you need higher resolution, up to 8K at 60 Hz.
In every case, the cables required are subtly different at each level, and it is important to obtain certified cables that are labeled PREMIUM for uses up through HDMI 2.0, and ULTRA for cables for HDMI 2.1
Apple talks about these steps in HDMI capability in four brackets, highest to lowest:
- Up to 8K at 60Hz (HDMI 2.1 at highest resolution)
- Up to 4K at 240Hz (HDMI 2.1 at highest refresh rate)
- Up to 4K at 60Hz (HDMI 2.0)
- Up to 4K at 30Hz (HDMI 1.4)
Connect to HDMI from your Mac - Apple Support