Build an object challenge part 2

Hi guys I have a few questions.

1. in the second part of the build an object challenge Dave McFarland created a new javaScript file to hold the program that will print the message out to the screen. How is it possible that the new javaScript file is able to access the names of the variables in the other js file when the two files aren't linked. Yes the two files are in the index.html file and run one after the other but the names of the variables in the first js file are called in the second js file when the two files aren't linked. He didn't mention that that was possible in previous lessons. How is the second js file able to use the names that are in the firls js file.
2. in the instructions video he told us that for example we created an array and named it students. To get to the first student object you'd use bracket notation and an index value of 0, like this students[0]. Then to get to the name of that student we would add on the name property like this students[0].name. however in the solution video, that's not how he did it. he first created a student variable and gave it a value of students[i] then he created a message string and concatenated each item in the object with the .name property. I assumed that the example he gave all goes on one line. like he did in the video.
3. why did he first create a message variable and gave it an empty string?

explanation video: https://teamtreehouse.com/library/javascript-loops-arrays-and-objects/tracking-data-using-objects/the-build-an-object-challenge-part-2

solution video: https://teamtreehouse.com/library/javascript-loops-arrays-and-objects/tracking-data-using-objects/the-build-an-object-challenge-part-2-solution