Sure, you draw the line at different levels of abstraction, but you still need to rewrite from scratch the thing you're learning. If you want to learn how a web

Sure, you draw the line at different levels of abstraction, but you still need to rewrite from scratch the thing you're learning. If you want to learn how a web server works, you better write one. You don't need to write the OS that it runs on, however, although doing so also informs certain aspects of web server design. Reading the documentation of an existing web server alone is only going to result in a superficial understanding of it.