Solar UV helps a little, but UVC (180-280 nm) is necessary to thoroughly kill many bacteria and viruses (including COVID) and UVC doesn't reach the Earth because the atmosphere absorbs it.
I think the idea that sunlight doesn't breakdown the virus comes from people trying to "cure" cases of covid-19 with sunshine, which yeah, that's not going ot work.
UVB does work but it takes longer than UVC. So long that if you try to use it to disinfect skin, it's likely to give you sunburn before it kills much of the virus.
Contrast Far-UVC (200-235 nm), which kills the virus quickly and yet does not seem to cause skin or corneal damage, despite being more energetic than UVB.
https://academic.oup.com/jid/article/222/2/214/5841129?login...
https://www.cedars-sinai.org/newsroom/study-reveals-ultravio...
I think the idea that sunlight doesn't breakdown the virus comes from people trying to "cure" cases of covid-19 with sunshine, which yeah, that's not going ot work.