Cannot install pip on Ubuntu 18.04 LTS

I am trying to install PyPI (pip) on my Linux/Ubuntu 18.04 LTS operating system without success.

The Linux distribution came pre-packaged with python 3.6 (as it appears in /usr/bin), but pip does not seem to be pre-installed as when I try to install a package from a terminal using pip install <package-name>, it replies:

Command 'pip' not found

I tried installing pip using the following:

sudo apt install python3-pip (source: https://www.tecmint.com/install-pip-in-linux/)

sudo apt install python3-venv python3-pip (source: https://packaging.python.org/guides/installing-using-linux-tools/#installing-pip-setuptools-wheel-with-linux-package-managers)

But everytime, although something is installed, when I try using pip from the command line, it still doesn't find it.

I did not try the method proposed in https://pip.pypa.io/en/stable/installing/ using get-pip.py because of the following message which appears on their website (I'm worried that this may cause a problem as I believe my Python install is managed by my operating system since it came pre-installed):

Warning: Be cautious if you are using a Python install that is managed by your operating system or another package manager. get-pip.py does not coordinate with those tools, and may leave your system in an inconsistent state.

Do you know have any idea what the problem could be (or what I'm doing wrong) and/or do you know if there are any other way to install pip?