Four Wires Inside each box you should discover four wires that connect to each 3-way switch.
The red should be terminated on the other side.
There should be a cable entering each box with a red, black, white, and ground.
If you get the impression we all kinda "make it up" in 3-way switch circuits -- you got it!
One minor correction:
Thinking about colors is useless.
So you might find cases where power goes to the light first, or other strangeness.
The incoming power wire yellow in this example has the three wires.
Because of this, the switch may also require a neutral wire.