The form builder allows you to create signup form for you to collect email addresses and have dumped into a list.
Generally I suggest to create a brand new list to keep those emails separate so you can see who is came from your website.
Does the "Select List" affect the subscription
code that is generated?
Yes code is generated which determines what list the email address entered via the form is sent to.
What about the blank line "Custom Signup Complete Url
That is if you want to send someone to a different page on signup instead of our default thank you page.
Will everyone that signs up get every newsletter?
You will need to select the list that the emails are sent to with the list builder when sending out a newsletter.