Fortification is to lands what Equipment is to creatures. Except for the difference regarding what type of permanent they can be attached to, Fortifications and Equipment follow the same rules.

If the fortified land produced more than one color of mana, you'll create a Junk token as long as the target creature is at least one of those colors.

You can't sacrifice a Junk token to pay multiple costs. For example, you can't sacrifice a Junk token to activate its own ability and also to activate the second ability of Junk Jet.

Some spells and abilities that create Junk tokens may require targets. If each target chosen is illegal as that spell or ability tries to resolve, it won't resolve and none of its effects happen. You won't create any Junk tokens.

You pay all costs and follow all normal timing rules for the card played from exile with a Junk token's ability. For example, if the exiled card is a land card, you may play it only during your main phase while the stack is empty.