Current lab equipment can detect extremely tiny levels of lead, far less than harmful levels.
But current government doctrine is that there is NO safe level for lead, so the standard becomes “no detectable level”, a goalpost that moves as equipment gets more sensitive.
How does lead get into food? By being absorbed by plants from the soil.
https://extension.umd.edu/resource/lead-garden-soils#
All Maryland soils contain naturally-occurring background levels of lead, typically less than 50 ppm.
See my prior post. There are natural levels of lead in soil.