What do I do with an iso-file/a CD-image?

On Windows 7 and newer and most desktop environments for Linux/Unix you should get "Burn on CD/DVD" or a similar menu item when right-clicking on the ISO-file, with which you can burn the image onto a blank disk.

On MAC OSX you can use the "disk utility" to burn an image to disk

Most burning software also offers an option for burning ISO-Images.

Instead of creating a CD or DVD you can also use the image to create a bootable USB-stick. You can use UNetbootin or LinuxLive for example to do so. (Wikipedia has a list of other tools)

How can I use this server as a package source?

The URLs from the HTTP- or FTP-links above will have to be put into the configuration of your respective package manager. The following sections contain example configs for some systems. You can find details on syntax and structure in your distributions manpages and online-documentation.

baseurl can be defined as one single as well as multiple URLs. For using multiple ones it's a good idea to use the fastestmirror-Plugin

[base]
name=CentOS-$releasever - Base
#mirrorlist=http://mirrorlist.centos.org/?release=$releasever&arch=$basearch&repo=os&infra=$infra
baseurl=http://mirror.centos.org/centos/$releasever/os/$basearch/
	http://linux.rz.rub.de/centos/$releasever/os/$basearch/
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-CentOS-

[sl]
name=Scientific Linux $slreleasever - $basearch
baseurl=http://ftp.scientificlinux.org/linux/scientific/$slreleasever/$basearch/os/
		http://ftp1.scientificlinux.org/linux/scientific/$slreleasever/$basearch/os/
		http://ftp2.scientificlinux.org/linux/scientific/$slreleasever/$basearch/os/
		http://linux.rz.rub.de/scientific/$slreleasever/$basearch/os/
#mirrorlist=http://ftp.scientificlinux.org/linux/scientific/mirrorlist/sl-base-7.txt
enabled=1
gpgcheck=1
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-sl file:///etc/pki/rpm-gpg/RPM-GPG-KEY-sl7

# main repo
deb http://linux.rub.de/debian/ bullseye main
deb-src http://linux.rub.de/debian/ bullseye main

# security patches	
deb http://security.debian.org/ bullseye-security main
deb-src http://security.debian.org/ bullseye-security main

# updates, previously known as 'volatile'
deb http://linux.rub.de/debian/ bullseye-updates main
deb-src http://linux.rub.de/debian/ bullseye-updates main

GENTOO_MIRRORS="http://linux.rz.ruhr-uni-bochum.de/gentoo-mirror"
SYNC="rsync://linux.rz.ruhr-uni-bochum.de/gentoo-portage"

http_proxy = http://www-cache.rz.ruhr-uni-bochum.de:80/
ftp_proxy  = http://www-cache.rz.ruhr-uni-bochum.de:80/
no_proxy   = ruhr-uni-bochum.de
use_proxy  = on

To make the server selectable in software-manager:

LOC:DE
http://ftp-stud.hs-esslingen.de/pub/Mirrors/packages.linuxmint.com/
http://ftp.fau.de/mint/packages/
http://linux-mint.froonix.org/
http://linuxmint-apt.killerhorse.eu/packages/
http://linux.rz.rub.de/linuxmint/packages/

# You only need to select one mirror and uncomment them. Please,
# ONLY ONE mirror can be uncommented each time.
# Slackware64-14.1
#----------------------------------------------------------------
http://linux.rz.rub.de/slackware/slackware64-14.1/

deb http://linux.rz.rub.de/ubuntu/ bionic main restricted
deb-src http://linux.rz.rub.de/ubuntu/ bionic main restricted

## Major bug fix updates produced after the final release of the
## distribution.
deb http://linux.rz.rub.de/ubuntu/ bionic-updates main restricted
deb-src http://linux.rz.rub.de/ubuntu/ bionic-updates main restricted

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu
## team. Also, please note that software in universe WILL NOT receive any
## review or updates from the Ubuntu security team.
deb http://linux.rz.rub.de/ubuntu/ bionic universe
deb-src http://linux.rz.rub.de/ubuntu/ bionic universe
deb http://linux.rz.rub.de/ubuntu/ bionic-updates universe
deb-src http://linux.rz.rub.de/ubuntu/ bionic-updates universe

## N.B. software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu 
## team, and may not be under a free licence. Please satisfy yourself as to 
## your rights to use the software. Also, please note that software in 
## multiverse WILL NOT receive any review or updates from the Ubuntu
## security team.
deb http://linux.rz.rub.de/ubuntu/ bionic multiverse
deb-src http://linux.rz.rub.de/ubuntu/ bionic multiverse
deb http://linux.rz.rub.de/ubuntu/ bionic-updates multiverse
deb-src http://linux.rz.rub.de/ubuntu/ bionic-updates multiverse

deb http://security.ubuntu.com/ubuntu bionic-security main restricted
deb-src http://security.ubuntu.com/ubuntu bionic-security main restricted
deb http://security.ubuntu.com/ubuntu bionic-security universe
deb-src http://security.ubuntu.com/ubuntu bionic-security universe
deb http://security.ubuntu.com/ubuntu bionic-security multiverse
deb-src http://security.ubuntu.com/ubuntu bionic-security multiverse