Hi!
Firstly, welcome! In terms of your questions:
1. The design is not automatically responsive as you would think from responsive web apps, however there are plans to make responsive design in future roll-outs. Responsive design is possible, however it requires manually creating that responsive element on each object. One of our forum members has created a Responsive Design Template and shared it within the Community App Sample if you'd like to take a look.
What I will say about the responsive side, is that if you design a horizontal (16:9) Power App, and open it on the Power Apps Android/iOS App, it will rotate it to best fit, so that it uses the most amount of screen space. This leads me to my next point:
2. The App will run regardless of platform - if the device has a browser, it can run from it by anyone who is licensed to open it within your organisation. For your mobile devices, you can download the Power Apps App (android), or Power Apps App (iOS) which will run any solutions created within your tenant that you create or have been shared with you.
3. Yes, you need an Office365 license that includes Power Apps, however, you can use lots of different data sources, such as Excel / SharePoint / CDS / SQL / ??
Hope this has been, helpful, feel free to ask any other questions you may have 😺
Thanks!
ManCat
