Fair question. First; it's okay to be a beginner. Second, there are a zillion tutorials on the web, but I would start with either MDN: learn web development or W3Schools: W3schools HTML
Today, we'll create a basic web page. That will put all the pieces together. Then build on that: add bits of HTML and CSS from the tutorials above, until you start feeling more comfortable.
A placeholder is shown to the user, but won't be sent to the back end. The value, possibly edited or replaced by the user, is sent to the back end.
It can, but you wouldn't bother. Why use a more general technique when a simpler, more convenient one is available. That would be like texting someone who is sitting next to you. (Wait, you do that?)
Using labels can improve the UI for everyone. Without:
With labels:
It goes to the place you specify in the ACTION attribute. If you omit this, it goes to the same server that provided the page you are on, with the same URL you are on. Sometimes, that's perfectly reasonable:
https://server.com/profile/edit
Yes! Every link you click on sends a GET request to the server. So you can just type an arbitrary URL into the location box to send a GET request to that server.
Every FORM that uses METHOD=GET also sends a GET request.
I'd rather not get into the details. Students often find it confusing. But a request consists of some lines of text (headers) and then, optionally, a blank line, and some more stuff.
If the second part is included, then the first part is the envelope, the second is the contents (body).
If the second part is omitted, then the first part is the postcard.