Prophet Muhammad (s) is, according to the Shia belief, a ulil azam prophet who is the last messenger amd the highest of all prophets and he is also an Imam. Similarly we see in surah baqarah that propeht Ibrahim (as) was a prophet and also an Imam. However we believe that there os a living present hujjah upon the earth at all times. So, after the martyrdom of the prophet Muhammad (s) this was Imam Ali (as) and the Imams from his progeny. The current proof of Allah on the earth now is Imam Mahdi (atf).
We see in Quran that every people will be called with their Imam.
يَوْمَ نَدْعُو كُلَّ أُنَاسٍ بِإِمَامِهِمْ ۖ فَمَنْ أُوتِيَ كِتَابَهُ بِيَمِينِهِ فَأُولَٰئِكَ يَقْرَءُونَ كِتَابَهُمْ وَلَا يُظْلَمُونَ فَتِيلًا – 17:71
(Remember) the day when We will call every people with their Imam; then whoever is given his book in his right hand, these shall read their book; and they shall not be dealt with a whit unjustly.