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The Dybbuk is a monster from Mordenkainen's Tome of Foes (p. 132) that can possess humanoid corpses if they're intact.

What happens if someone then casts any resurrection spell on the humanoid? Is the Dybbuk expelled, or is the humanoid just regularly possessed now by the Dybbuk?

A body possessed by a dybbuk is undead, not "dead"

Most "resurrection spells" are cast on a dead body of some sort. For example the specific spell resurrection requires you to do the following to work:

You touch a dead creature that has been dead for no more than a century, that didn’t die of old age, and that isn’t undead.

The spell raise dead also requires you to touch a dead creature (and similarly cannot return undead creatures to life). And reincarnate also requires you to touch a "dead humanoid".

All of the above spells would be complicated by the fact that there's a difference between a dead creature and an undead one. Jeremy Crawford has stated (in answering a different question):

The undead are creatures... A dead creature has no hit points and is an inanimate corpse.

More specifically, the Dybbuk's possession of a corpse explicitly does the following (Mordenkainen's Tome of Foes, p. 132):

The dybbuk is now effectively the possessed creature. Its type becomes undead, though it now looks alive, and it gains a number of
temporary hit points equal to the corpse's hit point maximum in life.

...

The possession lasts until the temporary hit points are lost (at which
point the body becomes a corpse once more) or the dybbuk ends its
possession using a bonus action

A possessed corpse is a creature, not a corpse: and it is undead, not dead. Thus, many resurrection spells cast on a corpse occupied by a dybbuk would not be cast on a valid target, and would thus simply fail.

Even true resurrection would not work if the corpse is currently possessed. The text of true resurrection states that it targets (PHB, p. 284, bold added):

a creature that has been dead for no longer than 200 years

The tense of "has been dead" is the present perfect tense, which indicates that the action began in the past and continues into the present. So the creature must be currently dead to be a valid target of the spell.

There is one spell which doesn't explicitly require a dead target: revivify only requires a creature that "has died" within the past minute. But revivify may not work as intended, since the "creature" you are touching is not the creature you wish to return to life (a humanoid) but rather a new creature (an undead). Jeremy Crawford has stated:

If you cast revivify on a zombie, the creature returns as a zombie. The spell doesn't change creature type.

So casting revivify on a dybbuk possessed corpse may simply add a hit point to the undead creature (in addition to the temporary hit points it already has).

Although your DM might decide, via the "rule of cool", that resurrection attempts cast on a dybbuk possessed corpse may work differently than attempts to resurrect typical undead creatures since the dybbuk is only a temporary occupant of the corpse, or simply because it would be fun to see the dybbuk expelled by holy magic. But via the Rules As Written, if you want to return a dybbuk possessed corpse to life, you will probably have to first eject the dybbuk from the corpse.