But in all seriousness, whatever you send to a company or complete a form, it's just text. And text is not expensive.
Open a notepad, hold down the button "a" for 60s which is the equivalent time one needs to complete a simple form, save it, and see how big it is. Should be around 4kB. It's easy to send 4kb across the web.
As for the email, that's to do with routers and DNS.
Now I'll leave out stuff like TCP and HTTP and all that crap and explain it simpler, I mean, I'll simplify a lot of it.
So your router works all the time. It "talks" to everything else around it so it can get a sense of where it is in the world in relation to everything else. This is very useful and it is what allows you to connect rapidly to any website you want. Because your router will know what paths to take. Think of it as a series of bridges, the router is the one that knows which bridges are the best to take for you to get to where you wanna get. so it knows what bridges collapse ever so often, which are reliable, which can take your load, etc. Well, modern ones know. Older ones are awful. so the router knows which route to take to get the data you want to you and to send the data you want sent to where you wanna send it.
Simple.
Now lets talk about DNS. DNS is a fancy term for a simple translation operation. It translates what you type in a browser, so say, myPage.com, into something it can really understand, and that is an IP address, as in, the IP address of myPage.com. This is why you don't have to type in 203.221.0.31 for instance, you just type myPage.com. How it does this the freaky stuff which I won't go into but your computer basically sends out a question "What is the IP address of myPage.com?" and the God of the Internet answer. And then you access that page after the router finds the best route for you to go on.
Now the reason you get emails almost instantly is because you think in seconds while a computer thinks in nanoseconds.
Remember what we talked about text being cheap to send because it's just 4kb of text? Add that to the fact that computers LOVE dealing with text, I mean, it's like Christmas for them. Handling all that form information is super simple for any computer really. even 10 years old ones.
Not only that, but you can't screw up a form validation if the person who programmed it knows what he's doing. Because you can put safeguards in the form itself in the webpage so that all the information you send is correct or at least where it should be. Long gone are the days where if a form asks you for a telephone number you could type myphonenumber and have it accept that. Now, it's just numbers. Same for email, names, whatever. There is a lot of validation going on in the web browser itself so that the server can have some certainty that what it gets is true.
Now when you connect to a webpage your computer does something called a "handshake" with the server hosting that page which is a 3-part discussion. (you) Syncronize, (server) syncronize-acknowledge and (you) acknowledge. With this you establish a connection. So the server now knows your number, literally, your IP number.
So when you that 4kb of text input into a form, the server does what it does with that inexpensive load of info in a milisecond and sends out a reply to your given email address because the router already did the job of finding the easiest path to that server and because your computer already established a connection to that server.
