Trying to get documentation about act command

"man apt" (which refers to "man apt-get" for more details) and "man dpkg" should answer your questions between them.

Thanks. Will try.

1. From man apt:
update (apt-get(8))
           update is used to download package information from all configured
           sources. Other commands operate on this data to e.g. perform
           package upgrades or search in and display details about all
           packages available for installation.
Pls point me in the direction of some documentation that would tell me what the -m in "apt update -m" means.
To anticipate a question, I know and use sudo.
2. Also I have not been able to locate
The APT User's guide in /usr/share/doc/apt-doc/, apt_preferences(5),
       the APT Howto.
Could you point me in the direction of documentation which will guide me to these 3 documents?
I have apt directory which contains changelog.gz, copyright, examples, and News.Debian.gz
3. Could you point me in the direction of some documentation which sheds some lights on the use of the number 8 in "apt-get(8)" and the number 5 in "apt_preferences(5)".
Thanks for your help.

at the beginning there were several different commands for the package management tasks (apt-get for downloading ans installing, apt-cache for listing cache information, apt-key for manipulating cryptographic keys, …), and now there is the idea to combine all these functions in one executable apt.

I suggest that you look at the man pages for apt-get for an option which is not explained in apt's man pages.

       -m, --ignore-missing, --fix-missing
           Ignore missing packages; if packages cannot be retrieved or fail the integrity check
           after retrieval (corrupted package files), hold back those packages and handle the
           result. Use of this option together with -f may produce an error in some situations.
           If a package is selected for installation (particularly if it is mentioned on the
           command line) and it could not be downloaded then it will be silently held back.
           Configuration Item: APT::Get::Fix-Missing.

Normally apt(-get) will refuse to install packages when one of the packages cannot be correctly downloaded. With that flag it will ignore such package and install all other packages as far as possible without breaking dependencies.

For the APT User's guide in /usr/share/doc/apt-doc/ you have to install the apt-doc package.

The numbers (5) and (8) are chapters in man pages
https://unix.stackexchange.com/questions/3586/what-do-the-numbers-in-a-man-page-mean

For your initial question:

* apt clean
delete the cache files for apt, such that all repository information is re-downloaded with the next command
* apt update -m
download repository information to your system, I am not sure whether the -m option is relevant at all
* dpkg --configure -a
Configure all packages that are only partially installed (see "man dpkg" for details)
* apt install -f
Download and install all packages that are necessary to bring the package management back into a consistent status
* apt dist-upgrade
Download and install all available packages, even uninstall packages when this is necessary to install an upgraded version of another package
* apt autoremove --purge
Uninstall all packages that are no more required as dependencies of other packages, and delete also their configuration files.