I have an article with many TikZ pictures. In order to make the code more clear and to save compilation time, I want to remove the TikZ code from the main document. I read that this is possible with the tikzlibrary "external". (for example see this question).

But I am not sure, where to write which commands?

What do I write in the file/the files which consists only of the code for the tikz pictures?

What and where do I write in the main document in order to import the pictures?

There are two questions hidden in your post, they have two answers that are basically orthogonal:

Speeding up compilation by using tikzexternalize:

You don't actually have to write them into separate files to save compilation time. You can leave your code almost unchanged, that's the beauty of it. This is the simplest possible setup with externalization

documentclassarticle
usepackagetikz
usetikzlibraryexternal
tikzexternalize[prefix=figures/]
begindocument
begintikzpicture 
 node real complex figure;
endtikzpicture
enddocument

This will only run if your LaTeX is set up to run with shell escape (see here for example)

Having tidy code, by writing TikZ code into separte files:

Of course you can also store the begintikzpicture...endtikzpicture code in an external .tex or .tikz file and use input to include it. But that is a matter of taste and does not effect compilation performance.

You could write

documentclassarticle
usepackagetikz
usetikzlibraryexternal
tikzexternalize[prefix=figures/]
begindocument
inputtikzfigure1.tikz
enddocument

and
into tikzfigure1.tikz:

begintikzpicture 
 node real complex figure;
endtikzpicture

I tend to define my own command to include tikz files, instead of using input in order to take care of one more thing:

newcommandinputtikz[1]%
 tikzsetnextfilename#1%
 input#1.tikz%

This will make sure that the externalization is based on the file name (instead of the order) so that it doesn't get confused if you change the orders of TikZ pictures in your document.