Question

why are virus considered as being
at the border line between living and nonliving things

Answer 1

when it is outside the body of any living beings,it acts as non-living thing but as it enters our body it becomes living

Answer 2

Viruses are considered the borderline or in-between living beings and nonliving beings as they act inert outside the host body or host cell and become active once again using a host machinery (compensating the hosts natural resources and minerals present in their body), once inside a host cell it multiplies independently to form several copies of itself and then assemble themselves to produce virus particles.
In all they are living beings when they get a host (through entrypoints) and non-living beings outside the host cell. So, they need a medium to reproduce. Hence, they have been marked as a borderline between the living and nonliving beings.