If I remember my Clancy books right a lot of modern military radars are powerful af. They CAN detect things like birds, terrain, falling bombs etc, but that level of sensitivity creates a ton of noise for operators to sift through.
So computers apply filters so only things that meet certain parameters (speed, size, altitude) are visible to the operator. In theory a jet flying vertically at 0 ground speed could be invisible on a radar screen until they move horizontally again. The radar can see it but it isn’t showing it.
All of this is made tf up btw correct me if I’m too non credible