My situation:

You have a machine that is not connected to the internet and you would like to install a package on the system. Both my offline and online machine are using exactly the same operating systems including its version.

Benefits:

Save bandwidth
If the machine you want to install packages on is either not connected or you have slow / unstable internet connection.

Warning: There are many dependencies for a package, the way I was able to perform a sanity check was to create a virtual machine that contained no installs, turned off the internet and created a shared folder between host and guest. Then I would try to install a package on the VM and it will provide the dependency errors if there are any. Removing the dependency errors simply requires you to download the package and install them in the right order. After successfully installing everything on the VM and tested the tools are all working, I would move all the downloaded packages to the actual offline machine through CD / USB.

apt

Online machine:

Download the dependencies in your current directory

apt-get download <PACKAGE_NAME>

Transfer over the directory.

Offline machine:

Install deb package in current directory

apt install ./package_name.deb

Check all package dependencies¹:

Note: Even using this tool I had missed a few dependencies due to the lack of tracing back on the dependencies. For example, while looking at the dependencies for python-venv, I did not look at the dependencies of the dependencies etc.

apt-cache depends <PACKAGE_NAME>

Example :

>>> apt-cache depends python3

python3
  PreDepends: python3-minimal
  Depends: python3.6
  Depends: libpython3-stdlib
  Suggests: python3-doc
  Suggests: python3-tk
  Suggests: python3-venv
  Replaces: python3-minimal

PreDepends:

This field is like Depends, except that it also forces dpkg to complete installation of the packages named before even starting the installation of the package which declares the pre-dependency.

Depends:

This declares an absolute dependency. A package will not be configured unless all of the packages listed in its Depends field have been correctly configured (unless there is a circular dependency as described above).

Suggests:

This is used to declare that one package may be more useful with one or more others. Using this field tells the packaging system and the user that the listed packages are related to this one and can perhaps enhance its usefulness, but that installing this one without them is perfectly reasonable.

PreDepends, Depends, Suggests, are retrieved from ²

yum

Using the yum downloader³

Python3 is not included in the built-in yum repositories, there is a community maintaining the upstream packages for centos⁴

Online machine:

Add the repository to our package manager:

yum install -y https://centos7.iuscommunity.org/ius-release.rpm
yum update

Install yum-utils for yumdownloader. Yumdownloader allows me to download the dependencies using --resolve.

yum install yum-utils

yumdownloader --resolve  <PACKAGE_NAME>

Generate metadata for our RPMs repository to make it a yum repository

createrepo -v <PATH_TO_PACKAGE_DIR>

Transfer over the directory

Offline machine:

Add the yum repository to the list of repositories on yum

vi /etc/yum.repos.d/localrepo.repo

[localrepo]
name=Awesome Repository
baseurl=file:///path/to/repo/folder
gpgcheck=0
enabled=1

Note: Use three slashes(///) in the baseurl.

Install rpm package located in your current directory.

yum install --disablerepo="*" --enablerepo="localrepo" <PACKAGE_NAME>

Where the package name is the same as the name you would use to install a package from the internet.
Tell yum to look for packages in the localrepo only.

https://www.ostechnix.com/download-rpm-package-dependencies-centos/

Method two, using yumdownloader

https://www.unixmen.com/setup-local-yum-repository-centos-7/

Ignore the ftp section
Focus on the “Build local repository” section

https://unix.stackexchange.com/questions/259640/how-to-use-yum-to-get-all-rpms-required-for-offline-use

Followed a similar process to the best answer