How do small airports make money?

Could you elaborate on the various revenue streams that small airports rely on to ensure their financial sustainability? I'm particularly interested in how they manage to compete with larger, more established airports that typically have a wider range of services and amenities. Do small airports primarily generate income through passenger fees, landing charges, or are there other innovative ways they generate revenue? Also, are there any specific challenges unique to small airports that they need to overcome in order to stay afloat financially?