My 9 year old loves this but I don't give him much screen time so he hasn't played it much.
I have it on my phone only right now. If I get it for my laptop, can I buy it once and put it on a couple of machines?
When you buy the game on the PC you can access it from any other PC because you have to sign into the main Minecraft server to play it. It doesn't download to your computer.
But certain files do, like the savegame files and any mods you download. So you can play it on more than one machine but you wont have the same game on every machine. You also have to remember that the PC minecraft world is infinite. There is no end to it at all. No final wall that you hit. There is a bottom floor - the bedrock - that you can dig to and I'm sure there is a limit to the height in the sky at some point, but east and west there is simply no end. On one of those websites that shows the size of planets and atoms and people and how everything relates to each other, the PC minecraft "world" is bigger than the planet Neptune in terms of quantifying the size of its data. So obviously you are't downloading that.
The XBox game is finite and has outer walls, but it is still so big with so much to do that you will never ever get to its end there either.