Below 10,000 feet MSL (mean sea level): A transponder is not required unless the aircraft is within 30 nautical miles of a Class B airport. At or above 10,000 feet MSL: A transponder with altitude reporting capability is required.
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However, if you wish to operate in class A, B, or C airspace, or at an altitude of over 10,000' MSL, or within a 30 nautical mile radius of the primary airport in class B airspace, you will need a transponder and altitude encoder (commonly referred to as "mode C").
Two-way radio communications and operational transponder are normally required for operations within Class C airspace, but operations without radio communications or transponder can be conducted by LOA, facility directive, or special arrangement with Class C airspace controlling facility.
You may fly without ADSB under the Class B or Class C shelf provided you remain outside of any Mode C veil. §91.225 tells you where you cannot fly. It says you cannot fly within the lateral boundaries of the Class B or C airspace or above the airspace.
A Squawk 7700 indicates an emergency. This can be an emergency of any kind. Pilots may input it into the transponder themselves – or when instructed to do so by ATC. As a result, ground control will know that the aircraft is dealing with a serious issue and needs help.