What is a local mirror of a debian apt repository? any simple explanation. Is it the same as ubuntu repository? what is the best way to create one? and is it mandatory to keep the same folder hierarchy as the original repository? or there is a way to configure that change?
when not to mirror
Before even bothering to start mirroring a whole repository, you should be aware of this:
I hope you mean simple as in simple wording, rather than short/compact.
Let's zoom out a bit, first. Debian and Ubuntu are very common Operating Systems, specifically, Ubuntu is derived from a Debian base. One of the key features of Debian and therefore also Ubuntu, is the package management. Repositories are a set of packages published in a certain way that the package management can easily understand the package list and verify the integrity of all of it. Repositories are copied all over the world for more efficient use of bandwidth as well as to have a higher availability. This continuous copying (or synchronisation) is also called mirroring.
Now, zooming back in on your question. You can do this mirroring operation yourself as well. This will pull in tens of Gigabytes of data just for the main repositories, but once copied, you'll have them local, at LAN speed! This is useful for deploying a big amount of machines and you don't want to rely on the internet for some reason (bandwidth, availability, etc.). You can mirror from a local mirror near you or pick one that has a lower delay.
If one refers to a Debian APT repository, this is ambiguous for either a repository for...
The format of the repositories is the same, speaking in terms of simple ones. Debian has some differences for differential updating package lists for example, but it doesn't mean it's incompatible.
If you do a simple mirror, you can pick another sublevel, e.g.