I was watching this video about the Tarrasque when I saw that its stomach fluids strip magic. I was wondering if a Wizard is swallowed by a Tarrasque, does the wizard lose his innate magic or is it only for artefacts/magic items?
Nothing, but its stomach acid can digest even the most powerful magic items.
Being swallowed by the tarrasque has no special effect on a wizard's ability to use magic, whether in D&D 5th edition (Monster Manual p.286) or earlier editions.
However, the tarrasque's stomach is canonically capable of digesting magic items, even certain powerful artifacts. D&D 5e Dungeon Master's Guide, p. 221, Destroying Artifacts, suggests this method:
The artifact must be swallowed and digested by the tarrasque or some other ancient creature.
This trait is given more detail in the D&D lore of earlier editions. Most specifically, in Dragon #359's Ecology of the Tarrasque, p.92, it is noted:
The middle stomach of a tarrasque is a churning, energetically pulsing fleshy cauldron of corrosive acid. Unique in all the planes, this fluid possesses a disjunctive property, capable of stripping the magical powers from consumed magics. Here everything breaks down.
The "disjunctive" property is most likely a reference to the spell Mordenkainen's disjunction, which strips magic items of their power.
Of course, the tarrasque's stomach acid will eventually prevent a wizard from casting spells by digesting the wizard.
Tarrasque stomach fluids do not affect a wizard's magic
The Tarrasque has an acidic stomach, but it doesn't strip away magic:
While swallowed, the creature is blinded and restrained, it has total cover against attacks and other effects outside the tarrasque, and it takes 56 (16d6) acid damage at the start of each of the tarrasque's turns.
I am unsure if previous editions had an effect like that, but the 5e Tarrasque does not.
Your Wizard will be blinded, restrained, and take acid damage - but it won't lose it's magic.
Always check sources :)
User G.Moylan astutely points out:
that video series, while interesting, pulls from many sources including those for older editions or from outside the actual game rules. They are handy for understand creature motivations or behaviors for campaign or combat considerations, but they shouldn't be taken as gospel for the 5th edition versions of these creatures.