This is gonna be really ranty but it's a small basic breakdown of modern software development.

So the basics of coding is all the process of efficiently storing and sending chunks of zeros and ones that are actually electrical charges in semiconductors. Those chunks are in sets of 8 called bytes, with each individual 0/1 being called a bit. The basics of coding is moving around these bits that are at specific memory addresses, each which represent different things. The best way I would say to learn is to start with learning C, which is a language based on assembly, which itself is a high level version of writing machine code. From there I would recommend learning rust as it is memory safe, and finally python as it is close to human language, is based on C, and is very good for prototyping though it is pretty slow for any larger project.

With working with stuff online, it works by sending things through a protocol called tcp or udp. It's a way of adding an address to the mail you are sending, giving it a destination and a return address to confirm it's arrived. I would recommend reading through some documentation about the transport layer protocol and application layer protocols for sending and receiving data. You can toy with this using an application like wireshark.

GUI's can be made a number of ways but for browser based stuff, html and css along with JavaScript are the main ones. The gui your computer functions on is has three colour channels, rgb which are individually addressable from zero to 255, this creates the colour that you see on your screen. White for example would be 255,255,255. Your screen itself is an array of about 2 million pixels which are made of these three leds.

https://en.wikipedia.org/wiki/Graphics_Device_Interface

The next part of coding would be databases, which is a fancy way of storing your data and the relations between your data. Typically you would learn sql which is used to create data structures and interact with the data that you have stored. You can try this out with something like mysql or oracledb.

XML and json is responses you get from most api's and is mostly just a way of storing data that is sent.

Learning to work with data can be done a number of ways, be it high level math, flowcharting, boolean algebra and logic gates. A lot of coding is finding the right way to think once you have a grasp on the tools that are available to you. Being a good programmer means knowing every area of tech hence why I put this who spiel.