In order to have a ridesharing policy with a standard car insurance company (such as Geico or Progressive), you have to have a personal policy already established. A personal policy is what the average driver in the US currently has.
Yes, typically it is cheaper to bundle your car and home insurance policies with one insurance company. Most companies will give you a multi-policy discount when you have more than one line of business with their company.
With the majority of companies, a prior insurance discount for auto coverage is based on being previously insured under an auto policy. Unfortunately most companies won't consider an existing motorcycle policy to be sufficient to qualify for a continuous insurance discount.
A golf cart is covered on a Special Lines insurance policy since it is a different type of risk from a traditional automobile. Unfortunately that means you won't be able to add it with your vehicle, but most insurance companies offer discounted rates for "bundling" your two policies.
The benefit of bundling auto and home insurance is that you receive a multi-policy discount. If your auto insurance company also underwrites your home insurance coverage then you might even have just one company handling all of your claims.