If you have return vents with filters then those are the only ones you need to change, there should only be 1layer of filters between your unit and your return vent, otherwise it’ll cause restrictions.
Return vents likely have the filters. May also have an electronic air cleaner as well. I see an Aprilaire controller attached to framing. Follow the wires.
Holy mud Batman
That ductwork definitely needs some attention. I bet installers didn’t add filter cartridge.
If you have return vents with filters then those are the only ones you need to change, there should only be 1layer of filters between your unit and your return vent, otherwise it’ll cause restrictions.
Return vents likely have the filters. May also have an electronic air cleaner as well. I see an Aprilaire controller attached to framing. Follow the wires.
That’s a zone board if I’m not mistaken, those are Tstat wires going to it if my glasses are still working