Imagine I’m listening to music from my phone via Bluetooth in my car.
Is it possible to say to a phone …”play blah blah” and it does it?
As I’m not really keen on spending too much money I’m looking at either an iphone 6\s or s galaxy s5.

I’d imagine so, given that saying ‘ok Google, take me home’ to mine in the car opens the satnav and starts directing me to the road I live on.

Yes. Most android phones will do this. It is far from perfect but does work

Yes as long as you’ve turned on all the feature, can be a bit of a battery drain having it always listening though.

Edit: shocking typing

Yes. Ive never used ok google or alexa but on my iphone if i hold the call button on the steering wheel it turns siri on and i can ask it to do stuff.

You need Siri (Apple only) or enable Google Assistant on Android, or can use Cortana (Microsoft’s thing, ties up with Cortana on the desktop also, runs on iOS and Android). There’s also Alexa (Amazon’s thing) which needs their shopping or music app.

Alexa is likely tied to Amazon Music and Siri to iTunes. Not sure what Google’s looks at. Cortana can be linked with various services including Spotify and also the remnants of Groove (no longer Spotify-like service, but it can still stream your own music collection if you dump it into OneDrive).