i totally agree with @NathanCu that starting with something and modifying it is a way to go. nathan said he uses firemote. i use this as a starting point (because i have fire tv) and extend it to control my yamaha amp and sony and lgtv’s:
i’d encourage you, like nathan, to start with something like this… pick either your tv or your streaming device as the starting point, and get the remote to control just the one thing.
then start adding code to change one of the buttons to do multiple things… personally i’d recommend replacing the power button from turning on/off the tv to turn on/off everything. in most of these canned remotes it lets you call into a service… i would put power on/off as a script… a script is a callable service.
in this way you can figure things out bit by bit.