Should I answer a revise decision saying that I will? Should I answer an EiC email that communicates a revise decision and sends me the reviews, saying that I will? Or just revise an upload the revised manuscript when it is ready?
Just submit the revisions when ready, according to whatever the procedures for that particular journal are. No need to bother the editor with an extra message that doesn't require any action from them.

Often, journals request revisions within some time interval; if you expect to need more time, for example if reviewer requests will require additional experiments, the journal will probably ask that you request an extension. Otherwise, no contact is necessary unless asked for until you send revisions.