Converting existing 3-way switches to smart switches
Based on the diagram (with yellow =&gt; white), it looks pretty straightforward, but actual pictures instead of drawings would confirm the details:
Box A:

Cable 1 = 3-way. Red &amp; Black = travelers. White = switched hot.
Cable 2 = Light fixture. Red = switched hot. Black = always hot for use in other stuff past the light fixture. White = neutral.
Cable 3 = Incoming power. Black = hot. White = neutral.
Cable 4 = Outside light. Black = switched hot. White = neutral.

Box B:

Red &amp; Black = travelers. White = switched hot.

For your smart switch, install it in Box A. You will probably need a neutral - just add it to the bunch of existing neutrals.