I am using Omnibus F4V3S FC which has built in LC for image filtering, i am confused how it works
Actually i am running a Runcam Split 3 micro which turns off itself, i want to use a step down buck which has a potentio + fixed output as well.
I am thinking to power my Cam board with external regulator and my AKK Race ranger VTX from Vbat.
How do i put the built in LC filter of my FC in use, how to wire?

you cant use the onboard filter if you wire the vtx and cam extarnal, an external source like a bec does mostly provide pretty clean power. matek has tiny nice becs.

todays cams and vtxs has mostly filters onboard by itself, i would give it a try with the bec or even directly to the lipo in (i believe both could handle battery voltage?)
add an external lc-filter if you get bad video thisway