system installation - Dual boot - what happens with "Install alongside windows" ??

I was directed here from https://www.reddit.com/r/Ubuntu/comments/5qowau/dual_boot_what_happens_with_install_alongside/

Here's the question I posted there:

I'm following on line guides on how to set up dual boot Windows 10 and Ubuntu. Here's a part I haven't found an answer to.

When I get these options:

* Install Ubuntu Alongside Windows
* Erase Disk and Install Ubuntu
* Something Else

I realize that choosing "Something else" will allow me to set up my swap, root, home, etc partitions as I wish. If I choose the first option though, are there default settings for swap, root, home, etc. that Ubuntu will use? How can I preview the actual disk setup that will be used if I choose the first option?

Also, I see that before starting the Ubuntu install, you need to free up disk space in Windows. That much I understand. But looking at some tutorials like this video (can't post more links here but see my reddit post), when you choose "install alongside windows" it then asks you to divide space between Windows and Ubuntu. See video at about 3:40. Why does it ask about dividing disk space here? If you don't use it all, will the remaining space stay unallocated? EDIT: Or does this option only show up if you didn't allocate free space? And if so, is there a preferred method (allocate space in Windows or do it during the install), and why is it preferred?

TY!