To create a private Ubuntu image, the commonly used tools are debmirror and apt-mirror.

As of today, these two tools have their own advantages and disadvantages.

debmirror

debmirror document

debmirror has many downloading methods, and it will compare the local file with the file on the server before
downloading so that only the missing files are downloaded. However, it does not seem to support multi-threaded
downloading. Debmirror is downloaded one by one, so the download speed is relatively slow.

apt-mirror

apt-mirror document

apt-mirror supports 20 threads to download at the same time by default. If you choose a suitable mirror station, the
download speed can reach the maximum of my network. However, it does not seem to make a comparison. Although it can
speed up the download start speed, sometimes some files may be lost, resulting in an incomplete mirror, so the client
cannot be updated.

how to write the config file

In order to download the complete package, we need to modify the configuration file in /etc/apt/mirror.list. Use this
format to ensure that the i386 package will be downloaded.

deb-amd64 http://mirrors.aliyun.com/ubuntu focal main restricted universe multiverse
deb-i386 http://mirrors.aliyun.com/ubuntu focal main restricted universe multiverse

Combine them together

Therefore, in order to achieve a balance between efficiency and completeness, we can combine apt-mirror and debmirror.
We can first use apt-mirror to download quickly, then we can use debmirror to check and download the missing packages.
Maybe this is the best way to download the Ubuntu mirror.