How does apt-get work and select the repository from which to install a package?

Question

linux

Hello, my problem is the following. The distribution is based on debian 7 wheezy, the repositories are all added to /etc/apt/sources.list , the first repository is the distribution itself, next comes from the debian repository, and so on.
Let's say I want to update the system, apt-get upgrade, it tells me that everything is OK, there is nothing new, but as soon as I put the debian repository on the first line in the sources.list file, when updating, it immediately finds all new packages.
Therefore, I can assume that all these packages have the latest version in the original repository. How can I set up the list of repositories so that it finds the latest packages from all the repositories in the list?

Answer 1

It's a bad idea to pull packages from different repositories. The way you found (rearrange the repositories) is the correct one.
If you collect packages from everywhere, you can get a glucodrome from incompatible packages instead of a working system.

Answer 2

https://wiki.debian.org/AptPreferences

Answer 3

You don't have a mention of the update command.
In your case, it doesn't matter what the sequence of the repositories is.
The most recent stable versions are installed by default.