Flying anywhere is dependent on several things and becomes complicated in large cities or where commercial traffic is located ... not related to Part 107 vs. hobby per se.
First, learn how to read current aeronautical charts published under the authority of the FAA. They tell you which class of airspace is around any given airport. Class B airspace is located around large major airports, such as San Francisco, while Class C for slightly smaller airports includes Oakland and Sacramento. Class D are even smaller airports. Passing a Part 107 is heavily dependent on knowing how to read most aspects of these charts.
The first circle around any of those (may be shaped different than a circle), will typically be in a radius of, for example, 5 miles starting at ground level, noted on the charts as "SFC" for "Surface", up to a height measured above mean sea level (MSL). The next surrounding same color circle (again, not always a circle) will show minimum to maximum heights for a particular airspace. BELOW this minimum is Class E and below that Class G where hobby or commercial UAV's can fly legally.
You do not need to notify anyone when flying in these areas, unless notified to NOT fly due to Temporary Flight Restrictions (TFR's) posted online by the FAA. Regarding calling ATC, they have the final say with FAA guidelines being the general next level of authorization. For example, I have been authorized to fly in a Class D airspace near my town for commercial work, but I have to notify and be authorized on a given day and time by local ATC, which has the authority to allow me or not to fly, based on current local conditions within their jurisdiction and within their airspace.
There are many other factors, but I hope this helps clarify. All small UAV's are subject to the same rules regarding airspace and TFR's.