If your Yamaha is going to be dedicated for outside use, then it already has an amplifier built in and will do just fine with your outdoor speakers. If the speakers CAN stay at the same volume on both sides, then you can skip the speaker selector/volume control and just plug 8 ohm speakers into the A and B connections of the Yamaha. If you want them to potentially be at different volumes, then get the speaker selector/volume control unit.
You didn't mention the setup you have outdoors, or the speakers you are planning to use, but just run some decent 14 gauge speaker wire to all the speakers and you should be fine and everything can be adjusted at the head end (receiver) location.
For Bluetooth, be aware that the range is EXTREMELY limited. Wherever you put your Bluetooth receiver, or even an extension antenna, will be a toss-up on whether it works or not. For reliability, most people now go with wi-fi enabled systems such as Miracast (Android) or AirPlay (Apple) for best results. Or, you can just plug an audio source into your Yamaha directly.
So, be specific about what you intend to use as your source. What you would like to use. Bluetooth is nice, but wi-fi is going to give you better range and greater reliability between a phone and the A/V receiver.
So, the Chromecast (miracast) audio device:
https://www.walmart.com/ip/Google-Chromecast-Audio/46784941
or a Apple Airport Express (AirPlay):
https://www.amazon.com/Apple-AirPort-Express-Station-MC414LL/dp/B008ALA2RC
Plus a 1/8" to RCA cable:
https://www.monoprice.com/product?p_id=9299&gclid=Cj0KCQjw0PTXBRCGARIsAKNYfG1QrNS4iuX7OQnqKuVEOPh4PR9WwRQVFQHGU0-XI6bfaOdScpUlcTkaAlxPEALw_wcB
Call it a done deal for higher reliability audio streaming from your mobile device.