You can't design it in full in one go, but you can design it and then incrementally update said design. Sadly many (companies) do not. But you can define the problem(s), the scope, the scale, and then design a solution appropriately to meet those needs (for a defined period of time). That's what distinguishes software engineering from hacking. They both have their place. Many companies claim to do the former but are mostly doing the latter. Software is still early in its life and as various kinds of system designs stabilize, so will the formalizations around what it means to be a software developer. Reading a book like Designing Data Intensive Application's you can't help but see those formalized topics budding.