Public transport in SoCal exists, for example, there are buses servicing Miramar Road where Legion HQ is located. However, it is notoriously bad and used by few.
If you are planning on staying in the area for a week or longer, we recommend renting a car.
There are several international as well as local companies that will rent you one.
Otherwise a lot of visitors rely on ridesharing services like Uber and Lyft.
Unless you are staying very much nearby in the Mira Mesa area, we do not recommend relying on scooters such as Bird, because of the distances involved.