IIRC in picard they explain it as a special subgroup of changelings the federation kept and experimented upon during and after the dominion war, improving their shapeshifting so that they could imitate internal organs too, and for longer periods of time before they had to revert, to make them better infiltrators I guess.

Turns out torture makes the subject quite resentful and the first of them (forgot the name, the weed smoking captain one) teaches others who are also seeking revenge its improved abilities. Downside: they lose their immortality.

Though the point still stands that they infiltrate earth and mimicked several prominent officers, as well as martok easily. So they really should be able to without torture, especially since they are basically timeless. They could have practiced shapeshifting for eons, and a nose is really not that difficult.