Submission to asset store: 'compiling' into dll

I was unable to find information about this: in the process of submitting a new paid asset to the asset store, the guidelines talk about the use of Unity's relevant tool to build the package that will be distributed, however if I understand correctly this means that the whole source of the asset scripts become open to the end user?

Besides the risk of code manipulation and consequent illegitimate requests of support, this possibility creates other unpleasant scenarios which can compromise the original work and its evolutions so I checked the asset store and noticed that many packages offer their content in standard c or javascript files -accompanied by one or more .dll files-

It is quite clear that these .dll files act as libraries that the project references... but I'm at a loss at how to produce them. Do you need visual studio or similar to code them into a dll, and then use 'declare' and such in your unity application, or is there a more streamlined way of doing this, perhaps directly through unity? The former method actually feels like reinventing the wheel since you obviously have to replicate data structures for communication between the library and the project?

Please help me understand how you create the dlls that encapsulate your asset's methods.

Thanks.