What is the Slack CLI and how is it used?

Question

I'm going to develop a Slack app.

I've followed an official Slack.com tutorial on Bolt to make a simple app and I kinda get what's going on there.

I've followed another tutorial for the Slack CLI that also makes an app and I don't really get why it works or what it does.

Confusion: Both can be used to 'make Slack apps' but they are different.

Question: What is each approach used for, what are the differences?

I've tried reading all the Slack documentation. I expected to find a comprehensive explanation on how Slack apps work / are made. Instead I'm just getting more and more confused.