A relay that can switch from one source to another - what's that called?
If the ground/common/negative side of your circuit can remain connected, then you need to "switch" only the "hot"/active side (voltage not disclosed) side. That can be done with a SPST (Single Pole, Single Throw) switch or relay.

However there are several things to be aware of if you want to do something like this:


CURRENT  Switching such a high current will require a large and expensive relay. These are becoming harder to get and more expensive as this kind of switching is done with semiconductors (transistors, thyristors, etc.) here in the 21st century.
VOLTAGE  Switching a high voltage will require essentially a "mains-rated" relay which will add even more to the price.
DC vs AC  Switching DC is more difficult than switching AC. Because AC naturally passes through 0V many times per second (50Hz or 60Hz) Breaking high-current DC requires a special kind of relay designed for such applications. This will add even more to the price.
CONTROL VOLTAGE  You will not find any relays that can switch 100A @ ???V which can be operated from 3.3V.  You will need probably a multi-stage relay driver circuit to allow the 3.3V logic-level to control another circuit with enough power to activate such a large relay.


You make it more difficult to answer your questions when you do not include the working voltage. It limits us to generic answers which may not be applicable to your situation.