Kindly refer to the following links):
The First Tashahhud | Sheikh Assim Al Hakeem
Salutation on Prophetﷺ (Durood) in 1st tashahhud or sit idle if imam takes too long
I finished tashahhud, Imam is yet to say the Salam, should I sit idle or say something
Reciting Durood / Salutations on Prophet ﷺ in the first tashahhud