I really know next to nothing about this topic, for example I have a commerical game here, it has one exe, one music.img and one data.img file and a couple of ini files etc, now these are not img files in the sense of mountable packed image files like iso, img, dmg rather I am assuming the developer just picked the extension img as a general file extension.

So these two files have all the assests stored in them, music, maps, textures, strings, etc, obioulsy in binary format. So how is this done, how to you package a bunch of file assests into one binary file like this, and why don't all devs do this, for example take a pk3 file which can obvioulsy be opened with winrar, so all the assests are available to see.

Any example, links or knowledge here?

How you do this is entirely up to the developer. There is no standard approach really....

On some occasions, it can be nothing more complicated than a zip or tar file.

Sometimes it may be a custom compressed file (probably using something like libZip to do the compression/decompression, and a custom header that will ensure it can't be opened as an archive).

Sometimes it may simply be a serialised memory dump containing all of the assets for a level.

And every so often, you may come across someone who uses encryption to add another layer of annoyance for people trying to rip the assets!

Whether or not a developer wants people to be able to navigate their asset packages, is going to depend largely on whether they care about protecting their art assets or not. For some games companies that support extensive modding in their games, they are likely to be much more open with their asset packages than a company that does not support modding. Ultimately there is no company wide standard governing this....

... So how is this done, how to you package a bunch of file assests into one binary file like this ...

As Rob has mentioned, the solutions are numerous. The custom package file format I use has some header, followed by a table of content, followed by so-called load units, followed by streamable data.

The table of content has an entry for each single resource stored within the package. Each entry also stores the relative address and length of the load unit which contains the resource. A load unit may contain a single resource or a couple of resources. The latter is called a "bundle" and itself has a kind of table of content with back references to the package table of content. Bundles are fine for resources that should be loaded together (e.g. all the meshes and textures of a model). The resource is at least the description of what it is, but may also contain the resource data. If not, or if only partly, then the resource data or additional resource data is placed as streamable data towards the end of the package file. For example, textures may be stored with the low levels of detail embedded and the high levels of detail as streaming data elsewhere.

How about you put all of those into a zip using the zlib ? it has one of the best licenses over in the wild; and maybe it just solves exactly your packing problem, and provides compression as a cherry on top of the cake.

A windows-only solution is to use resources (.rc files) and access
then with FindResource(); LoadResource(); LockResource(); calls.

I don't think anyone would want to run a 4GB executable file, as resources can be huge.

Today's PC games are 64-bit and can have resources over 30 GB. But, my Internet connection hasn't changed, so I spent a week
just downloading it!

A windows-only solution is to use resources (.rc files) and access
then with FindResource(); LoadResource(); LockResource(); calls.

I don't think anyone would want to run a 4GB executable file, as resources can be huge.
Today's PC games are 64-bit and can have resources over 30 GB. But, my Internet connection hasn't changed, so I spent a week
just downloading it!