You can Sync the YouTube TV channels to the Fire TV channel guide. However this is a free feature, so not guaranteed to work all the time. If you are having issues its recommended to open the YouTube TV app directly instead.
On the Fire TV, go to Settings > Live TV > Sync Sources > YouTube TV
After it's done, YouTube TV will load.
To see the channels, from the Home screen on the Fire TV go to Live > Down 2 clicks to Recent Channels > Left to Guide > Select
The guide will be laid out with the Favorite Channels up top (if you have them), Fire TV Recast (Antenna) channels next (if you have an antenna/Recast) and if you scroll down through those, you will see all of the YouTube TV channels below that.
You can mark Fire TV Recast channels AND YouTube TV channels as favorites, and they will all show up on top of the Fire TV Channel Guide in the Favorite Channels section together. To mark channels as favorites, scroll down to the channel in the guide, press the Options button (three lines) on the remote, and select "Add to Favorite Channels."
NOTE: Favorites from inside the YouTube TV app are not in-sync with favorites from the Fire TV channel Guide nor will recordings from YouTube TV show up in the DVR section on the Fire TV.