https://en.m.wikipedia.org/wiki/Noise_(video)

There are many sources of electromagnetic noise which cause the characteristic display patterns of static. Atmospheric sources of noise are the most ubiquitous, and include electromagnetic signals prompted by cosmic microwave background radiation,[1] or more localized radio wave noise from nearby electronic devices.[2]

The display device itself is also a source of noise, due in part to thermal noise produced by the inner electronics. Most of this noise comes from the first transistor the antenna is attached to.[2]