Home Assistant has no user interface on the device that it runs on, other than a command line interface. for configuring it, you would need to go to a webpage that Home Assistant provides.
But if Home Assistant does not connect to anything else, your mobile phone would not be able to reach it.
While in theory you could create a wifi hotspot on your mobile, and have Home Assistant connect to it, Home Assistant would be on the internet fully unprotected, so you would not ever want to do that. But also, Home Assistant runs 24/7, so that too would be a challenge when HA connects to your phone. It would expect the phone to be always there.
So getting a wifi router and connect the Home Assistant green to it would be the way to go. It could be a pretty cheap one. I would connect HA green with an ethernet cable. Then your mobile could connect to the wifi to access Home Assistant.
But there’s another thing I’m not sure about, and that is that Home Assistant might want to reach out to get the current time on boot, so I think that the wifi router should also provide an ntp server. I’m not sure how it reacts when that is not available. EDIT: I see HA green has a real time clock, so that may only be the very first time, or maybe not at all.
And you won’t be able to update the device without internet.I doubt if you can get that working through a mobile phone in such a setup.
Once setup, you might be able to convert your HA instance itself to a wifi hotspot using an addon like this one, and do away with the external wifi hotspot:
So if you’d be able to set things up in a place with a lan and wifi, and then move it to where you want it, it might be feasible. The wifi would allow you the access the ui on your mobile to configure it when you are near.
Note that I have never tried anything like this, so I might get some of these things wrong. It is not your typical installation, so it is hard to get experienced help I guess.