BT.709 (also written "Rec. 709") is the colour space of every HD video — full stop. Standardised by the International Telecommunication Union in 1990, it defined what red, green, blue, the white point and the brightness curve mean across HD broadcast, Blu-ray and all streaming services up to and including 1080p. If a video isn't tagged otherwise, players assume BT.709, and they're right ~95 % of the time.
The palette is not enormous by modern standards: BT.709 can represent about 35.9 % of all colours visible to the human eye (measured against the CIE 1931 reference). That sounds modest, but it covers the great majority of natural photography and was a major step up from the SD-era standards before it. Almost any current TV can display the full BT.709 palette without effort — coverage is essentially 100 % on consumer hardware.
In practice: pick BT.709 by default for all SDR HD content, including most live streaming, corporate video, YouTube uploads, training platforms, web embeds. Only step up to BT.2020 if you're actually producing 4K HDR — and even then, the BT.709 version is usually still part of the delivery ladder, because the largest single chunk of your audience still watches on HD-only devices.
