I'm no expert either, the wife has XM-Sirrus in the car, I piggyback( I'll explain later) on her subscription with my GW.
a) "Steaming" is a generic term IMO, most often "streaming" refers to receiving music via your data plan on your smart phone, there are "services" (like radio stations) that "stream" music via the internet using the "data" portion of the cell phone (like when you "google" something on your phone, you are using your data plan)... a popular provider of music is "pandora"...it has both "free" ( with commercials) as well as pay for music ( a subscription) without commercials ( and NO your new unit from amazon will not work with pandora). Streaming with XM radio ( satellite provided) just means that you have a connection with the sky and it sends the signal to earth ...you will lose a signal when going under a bridge, in a tunnel, and sometimes when the satellite is "on the horizon" and the angle to your receiver is too "flat" ( 180 degs).
b) Since you are buying a "unit" I assume that the XM receiver is not built in to your bike. you'll have to figure out a way to power it (12VDC) and how to get the audio signal to the speakers/intercom ( AUX port)...the "head" units have many buttons and a display , think of it as a typical car radio with presets and volume controls, read the instructions and explore all the different types of music/dialog you can listen to. (I'm a gear head, and one of my favorites while on a long rides is to find "car talk" which is on the NPR channel via XM ...the two mechanics ( Click and Clack) are a crack up (1 of the original mechanics, Tom passed away in 2014)...I laugh so hard sometimes that I start crying.
c) I have a gold wing with XM capabilities and navigation, and you can "buy" weather and traffic reports from the XM provider ...the weather reports will cut into the audio and announce "rain has been detected in your area" ( duh, I can see the clouds), and it will cause the map display on navigation to change colors on which roads are jammed (red)and which roads are clear (blue), it also announces "traffic ahead". I "piggy back" those services on my wife's account located in the car. ( you may want those services (weather and traffic) when you "sign up" and pay for your service.
d) billing can be a hassle as they take your credit card and too often automatically bill you each year (with an increase?) ...there are many other posts dealing with billing..you can research those on this site.
Hope this helps
"Experience" is a tough teacher, as you get the test first and the lesson later...