That article is outdated, most likely written at a time where Python 3.8 was not yet available on the stable Ubuntu repositories.

You can install the latest Python available with just apt install python3 (3.9 at the moment).
If you really want 3.8, I suggest you take a look at the deadsnakes PPA. It contains an archive of all Python packages for Ubuntu, and also newer versions that haven't hit Ubuntu stable yet (like 3.10).

You can add this PPA and install Python 3.8 with just three commands:

add-apt-repository ppa:deadsnakes/ppa
apt update
apt install python3.8

Regarding your other two bullet points:

• Yesn't. The thing is that, by default, /usr/bin is included in your PATH environment variable, which what enables you to just type python ... instead of providing the absolute path for the binary. It is also expected by some scripts, I suppose.
• You could, and you could also symlink it. The reason why it wasn't put there already is because the article used altinstall, which prevents exactly that. It is way more preferable that you install stuff through your package manager, however.