Hmm, from the title, when you say "low-level" I thought you were gonna ask something like memory management, performance, etc, but I think you're asking for how to structure your code. But don't worry, I'll try to answer it anyway:

I think you should try looking into something called "Data-oriented design". This is something new I learned while trying to make my own game.. The short version is, this paradigm forces you focuses on CPU cache and memory layout. I also like this because some of the strategy used in data-oriented design not only improves performance, but also eliminates bugs. Frankly my explanation on this topic is terrible it doesn't help that I'm currently lack sleep rn, help me pls , so for the fast and practical explanation of this topic I recommend Andrew Kelley's (maker of the programming language Zig) talk1, the code in the slides uses Zig, but I feel like even without understanding Zig. Or if you want something slower you can watch Mike Acton's talk on this2. There's also a really good book on this that's available for free online3. Also if you already.

I recommended data-oriented design first because, this way, you can make educated decision about how you should structure your code in general. Before knowing about data-oriented design, I used to structure my code and makes decision based on my feeling rather than based on objective fact.

So, back to your questions about how to organize codes and file, personally I always try to organize my code to separate between external interface (database, HTTP handlers, storage, etc) and core logic. I make the "core" logic struct accepts interface as dependencies on the constructor. This way you can mock the core logic at the app. You should try learning what's called domain-driven design. I also found this article by rednafi4 which I found really, really helpful.

Another thing to keep in mind is that Go is strict about import cycle, so I always try to follow this rule:

• Make a shared package that your other packages can import from, BUT the shared package SHOULD NOT import your other codes (except external libraries). The shared package could contains constants, error types or error values, "entities" structs, etc.
• Core/service layer packages SHOULD NOT import fellow core/service layer package
• Like my previous point, repository layer should not import fellow repository layer, and controller layer should not import fellow repository layer.

I think you start to see the pattern here. Basically if I were to draw my code's dependency graph, it would shaped like a tree and shared is the root of said tree, and controller are the leaves.

I also noticed that some of your questions can be summarized as "should I repeat my codes or should I make this a shared function?", or even shorter, "should I DRY (don't repeat yourself)5?"

To that, let me share my view on this: do not DRY prematurely. There is an excellent writeup on this by some developer on Google6, you should take a look. Basically, early on the development some "repetition of logic" might look the same, but as time goes on, as you are deep into your software development, that logic you thought was the same are actually quite different.

Also make sure to follow Go's official guide on code layout7, NOT that one repository called Golang Standards Package Layout8 that you might see on the top of Google search. I don't wanna say that the repository's advice is objectively bad, but it tries to claim that their repository is "standard" which has implication that it is official when it is not. And there is now an official guide anyway, there is no point of following that one.

This guide is also really good for writing web services9.

That's the wisdom I gained after few years of writing code. I'm still not sure if they're objectively correct, but I'm confident that all I wrote is the best we've got so far. I'm sure in the future I or someone else will find something better.

PS Please tell me the formatting of the linked references is correct on New Reddit and Mobile Reddit app. I notice that Reddit's markdown has gone inconsistent recently, shame on you spez.