Learn Spanish (Spain): Este móvil no es mío, es el de mi hermano. - This mobile phone is not mine, it is my brother's.

Mi means "my" and must go right before a noun, as in mi hermano. Mío means "mine". Since it stands alone after the verb without a noun right next to it, you must use the long-form possessive mío.

What exactly does el de mean here?

It translates to "the one of" or in this case, "my brother's". To avoid repeating the noun (es el móvil de mi hermano), Spanish simply uses the definite article el ("the one") matching the masculine singular móvil, followed by de ("of").

Why is it este móvil and not esto móvil?

Esto is a neutral word used for "this" only when you do not know what an object is, or when referring to a general idea. When "this" goes directly in front of a known masculine singular noun like móvil, you must always use este.

How would the sentence change if we were talking about a feminine object, like a tablet (una tableta)?

All the masculine words pointing to the phone would switch to feminine to match the tablet: Esta tableta no es mía, es la de mi hermano.