## Fireball ignores total cover as long as there is a corner within reach of the spell ##
Fireball:

> A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame. Each creature in a 20-foot-radius sphere centered on that point must make a Dexterity saving throw. A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one. **The fire spreads around corners**. It ignites flammable objects in the area that aren’t being worn or carried.

And this is the area of effect rulling:

> A spell’s Effect expands in straight lines from the point of Origin. If no unblocked straight line extends from the point of Origin to a Location within the area of Effect, that Location isn’t included in the spell’s area. To block one of these imaginary lines, an obstruction must provide total cover.

The fireball text allow it to circunvent some limitations of an area of effect spell, as long as there is a corner to expand. So a target behind a wall with a corner within the radius of fireball will still be affected. If there are no corners (the character is inside a locked building, for example) the fireball area of effect will not include past the cover.

Froghemoth bite action:
>Bite. Melee Weapon Attack: +10 to hit, reach 5 ft., one target. Hit: 28 (4d10 + 6) piercing damage and the target is swallowed if it is a Medium or smaller creature. **A swallowed creature is blinded and restrained, has total cover against attacks and other effects outside the froghemoth**, and takes 14 (4d6) acid damage at the start of each of the froghemoth’s turns. The froghemoth can hold up to 2 Medium or smaller creatures in its gullet. If the froghemoth takes 30 damage or more during a single turn from a creature inside it, the froghemoth must succeed on a DC 20 Constitution saving throw at the end of its turn or disgorge all swallowed creatures, each of which falls prone within 20 feet of the froghemoth. If the froghemoth dies, a swallowed creature is no longer restrained by it and can escape from the corpse using 10 feet of movement, exiting prone.

While inside the froghemoth, the character has total cover against anything outside the froghemoth itself. The froghmetoh is all around that creature, there are no corners to expand here. The fireball does not affect the swallowed creature.