You're right to be confused. What you're dealing with here is one system (Airtable's formula field), creating instructions for another system (URL parameters), that can be used to provide information to a third system (Airtable forms). I only really covered one of these systems, the Airtable formula field. I think the best way to answer your questions is to explain why these systems are being used in the first place.
You could get the same outcome for your data by sending everyone the same form with a name field, collecting their responses and then manually going through each one and updating the correct person's information. This would work but would be slow, labour-intensive and could still generate errors e.g. if two people have the same name, or if someone replies with a false name.
To automate the process and (somewhat) protect against errors and malicious responses, you can give each person their own unique form with a unique identifier. When their response comes back you (or the computer) can look at the unique identifier and know that it came from the person it was supposed to.
The unique identifier in this case is the Airtable record ID. Every time you add a new row or record to a table on Airtable, Airtable automatically and instantly creates a string of letters and numbers to identify it. That string is guaranteed to be unique to that record and will not change even if you change all the other fields for that record. That way we could have to people called Mohamed Salah, for example, both aged 30, both 2m tall but with their own unique ID strings.
So we want to give each person their own form to fill in, with their unique ID attached to it in some way, so that we know for sure, who filled out the form when it comes back. A good way to do this, and the way you're using in your code, is with URL parameters. The URL is the address to the form on the internet. That's everything in your code from "https://airtable.com/shrPd1axJx1k3Yg6u/" up until the first "?". After that it's all parameters.
Parameters are key value pairs, where you describe an attribute and then follow it with the data for that attribute. In the case of your question 1) we have "?" which just marks the start of a parameter (also called a query string) followed by "prefill_Guest%20ID" which is the key, an "=" which separates the key and the value and the finally you would get the value. If we want to add more parameters, we just add an "&" and start the pattern again.
So to explain the parameter in your question 1) the key describes the info you're about to receive in the value, in this case it's the Guest's unique ID that's mentioned above. So you could create a unique form for each person by going down the list of guests, one at a time, look-up the guest's unique Airtable ID, and add it to the end of the form URL in a parameter. For example the first Mo Salah has a unique ID of '123456abcdef', so his unique form URL would be https://airtable.com/shrPd1axJx1k3Yg6u?prefill_Guest%20ID=123456abcdef
But that would take ages and would require you to repeat the process every time a new person is added. So instead you can have Airtable create the URL, with the unique parameters, automatically. Airtable already knows what the unique ID is for each person, you just need to tell it where to look and add it on to the end of the form URL, which will be the same for everyone.
So your code is a set of instructions to automatically create a URL, which contains instructions to tell Airtable, for sure, who is responding to the form. You're using one system, to create instructions for another system, to feed into another system.
To specifically address your question 2), these different systems interpret the same characters and symbols in different ways. The Airtable formula field treats an & as an instruction to join two strings together. But in this case you don't want that to happen, instead you want the & to appear in the URL because it's doing the job of separating two parameters. So it goes inside the " " and Airtable ignores it as an instruction.
In your question 3) the text "&hide_Guest%20ID=true" has a function in the URL parameter (I'm not familiar with what it does but I assume it hides a field on the form that would otherwise show the Guest ID), but is meaningless to your formula field. So when creating the formula, you place it in a string and it passes through to the output, uninterpreted, to be used in the URL.
This is a good little article about URL parameters if you want to learn more about what you can do with them. Feel free to ask any more questions - I'm enjoying writing the responses.
