The outward dress of the clergy is really up to the local bishop. Most bishops do not interfere in this matter.
Regarding seminarians, that’s up to the Bishop and the Rector of the seminary.
In the case of religious the outward dress is up to the Constitutions of the religious community. It is up to the major superior to dispense or enforce it. No bishop can intefere in these matters. The most a bishop can do is ask the major superior for what he would like to see in his diocese. Even then, the major superior is not bound to comply, unless the religious is assigned to work under the bishop, such as at a diocesan parish. Also, the religious themselves may request a transfer out of a diocese where they disagree with the bishop’s request and the transfer must be granted by the major superior, because the Constitution overrules the bishop.
Whether one is speaking abour religious or secular priests (remembering that not all religious are priests) climate and culture may also influence the bishop or the major superior in their policies regarding dress.
For example, in Germany only religious Brothers and Priests wore Roman collars until after Vatican II. Now secular priests wear them too. Secular priests wore black suits, white shirt and tie. They did not have a tradition of Roman collar. It was rare to see a German cleric in a Roman collar, though some did. Father Joseph Ratzinger always wore a shirt and tie. His brother Fr. George Ratzinger seemed to like the Roman collar.
In Mexico, the government does not allow any priest or religious to wear religious garb outside of Church property except for Franciscans. In Cuba the only religioius who may wear religious garb are the Daughters of Charity and the Papal Nuncio. In China, no secular priest or religioius wears any distinctive garb in private or public, except for the Chinese National Church which is ruled by the State.
There are some religious communities that wear cassocks instead of a habit, such as the Claretians. Their cassock includes the short shoulder cape and the sash, regardles of whether the religious is ordained or a brother. It is the identifying mark of the Congregation.
Not everyone who wears a cassock or Roman collar is a priest. The older mendicant religious and the monastic religious of course wear the older habits or a Roman collar, but never a cassock.
Because of civil law, climate, culture and other circumstances, there is no universal rule. Bishops and Major religious superiors make the judgement call according to circumstances.