assalamualaikum sheikh, here when they pray taraweeh they would make dua after 4 rakahs, and also after witr, is this also a bidah? and what should we reply when they say the the madhabs allow it, should we just stay silent?

Ustadh AbdulAziz al-Haqqan

Dua in unison after the salah is a bid'ah as the ulama have mentioned.