Learn French: Il a préparé des desserts originaux. - He prepared some original desserts.

Most masculine adjectives that end in -al in the singular change to -aux in the plural. Since dessert is a masculine word, the singular adjective original changes to originaux to match the plural des desserts.

Would the ending still be -aux if he made original tarts (des tartes)?

No, the -aux rule only applies to masculine words! Because une tarte is feminine, you simply add an -s to the feminine singular form of the adjective (originale). So it would be des tartes originales.

Why does originaux come after desserts? In English, we say 'original desserts'.

In French, most adjectives come after the noun they describe. Only a specific group of common, short adjectives (often remembered by the acronym BAGS: Beauty, Age, Goodness, and Size) come before the noun. Because original doesn't fit into those categories, it is placed after the noun.

Why do we need the a in Il a préparé?

This sentence uses the past tense, called the passé composé. It is built using two parts: a helper verb (here, a, meaning 'has') and the past participle (préparé). You need both words together to say 'he prepared' or 'he has prepared'.