In Spider-Man: No Way Home feature film, Peter goes to see Doctor Strange, the Sorcerer Supreme, to get his help. However, Peter (and fans) are shocked to find that Stephen Strange is no longer the titleholder - it's Wong. In his absence, Wong took on the mantle in order to keep the Sanctum Sanatorium running.
With Multiverse of Madness making its debut, this debacle has become a bit of a joke, not only between Strange and Wong, but to fans as well. All jokes aside, this does beg the question of how does one become the Sorcerer Supreme? The answer has been explored in-depth in the comic books.
According to established Marvel canon, it seems that the Sorcerer Supreme is more of an office rather than a title of power. It is given to a practitioner of the mystic or magical arts whose skills and command over magical energies exceeds any being in a given dimension. This is a position that must never be filled by one person permanently, which is why there have been many who've had the title. The first mystical being to hold the title was Agamotto. Past figurers known to have held the title include The Queen of Sheba, Merlin, Isaac Newton, and the Ancient One.
The title is not permanent, and the Sorcerer Supreme is subject to change as seen fit by the Vishanti. The Vishanti are a collection of three powerful beings that have protected the Earth for eons. They bestow the title onto a fit candidate, and it is through them that the Sorcerer Supreme gains additional powers. If the person becomes unfit or unworthy, the Vishanti take away the title and bestow it on to someone else.
Candidates for the title must display immense control and power over the mystic arts. They don't need to practice a specific religion nor come from a specific background in order to be considered. They do, however, need to be willing to protect the earth and all its realms at all costs, and serve the Vishanti.
There are other ways someone can receive the title of Sorcerer Supreme. If the previous titleholder is killed by someone, that person can receive the title, but they don't gain the objects, powers, or access to exclusive knowledge automatically. A person can “bestow” or “lose” the mantle either by not being worthy or if it’s passed down by the former Sorcerer Supreme. This has happened to Doctor Strange many times, but he's managed to earn it back one way or another.
In the MCU, Strange obtained the title after the death of the Ancient One. That doesn’t stray too far from the comics. Stephen does obtain it after being ordered (by the Ancient One himself) to kill him in order to prevent an ancient being from taking over. Later, he competes in the Trials of Vishanti to officially obtain the title.
Strange isn’t the only one who’s held the title of Sorcerer Supreme. In fact, it’s been taken from him several times. At one point, Jericho Drumm, aka Brother Voodoo was in possession of the Eye and Strange's garb. Through the Eye, he was shown all the potential candidates to become the next Sorcerer Supreme, but ultimately, it was given to Brother Voodoo.
An unlikely character who's held the title would be the god of mischief himself, Loki. After becoming aware of some potential threats, he deemed that Doctor Strange, the Sorcerer Supreme at the time, was unfit to handle them. So, to challenge him, he cast an illusion of the Vishanti and tricked Strange into handing over the mantle. Loki was given the Sanctum Sanatorium and repaired the Cloak of Levitation using Asgardian thread. Loki spent his time as Sorcerer Supreme obsessing over a spell called Exile of Singhsoon, which would transfer all of the magic on Earth to him.
Strange was able to stop this plot and regain the mantle after teaming up with Loki to recapture the Void, which Strange had tried to use to stop Loki in the first place. Loki revealed his intentions of only preparing Strange for threats by pushing Stephen's boundaries and forcing him to break the rules. Stange was not happy about this, mostly because the entire situation had ended in the death of his dog, Bats.
Dr. Strange isn’t the Sorcerer Supreme in all realities. In the Dark Dimension, that title goes to Strange’s former lover, Clea. Illyana Rasputin, aka Magik, holds the title in her home world of Limbo. There are at least ten or so realms outside of Earth's, each with their own Sorcerer Supreme to protect it.
All jokes aside Wong was given the title by default whenever Strange was dusted and presumed dead. He does fit the necessary qualifications. Perhaps an event will occur between a powerful threat and Wong in which he gives his title back to Stephen. Or maybe Stephen won’t obtain the title. One can be a master of the mystic arts without the title of Sorcerer Supreme to spice it up.