According to a fatwa on IslamWeb (run by Qatar’s Islamic Affairs Ministry), the nikāḥ ceremony is simply this: the man, woman, her guardian and two witnesses should gather together (this can be done over a video call if some of these people are not living close to each other). The witnesses must be respected members of the community and known to be good Muslims. The woman’s guardian says: “[woman’s name] is your wife.” The man then says: “I accept her as a wife.” That is it. They are now Islamically engaged.

From Islamic law’s perspective there is no need for anything else. But many countries pass laws that require marriages to be registered with the government, so the imams who usually oversee these ceremonies fill out forms and submit them to the government, or ask the couple to first get a civil marriage certificate before accepting to perform the ceremony. But the ceremony does not require an imam, it is just traditional to have an imam since it makes it feel official and proper. Some Muslim cultures have no conception of a nikāḥ that does not include an imam.

Sources:

• Fatwa from IslamWeb (Arabic PDF)