I did this video for another reason (launchers etc.)
but it will answer your questions i think
It depends what version your playing with , the one i have is a mix and match but its panelled and works
but you may have a different version that you have to edit the theme scripts ?
they both work the same though
any of the button icons can be changed so youtube icon can be changed say for syncler (see video below)
you then assign the path to what the icon does when pressed ?
in the example video i have set Sparkle to open when live tv is pressed , then the bottom icons i have set to vod apps
but they can be set to anything , this is something i have been playing with for two years , also consider i think it comes with 20 themes and each theme can have different buttons and willdo different things when pressed ? you can for example have the current button for tv guide open youtube , just change the icon and your good to go.
You may think ohh wow! but ask yourself do you really want to be launching apps within another app ? it looks good but on nvidai shield i just press the stop button twice to clear any apps in memory and open another takes about half a second. There are a lot of launchers out there but there is not one that i will use in place of the android tv front end they are just not good enough , hopefully someone will come up with a way to kill those rows of movies and series for sale from amazon i think they are on amazon devices as well ?