SQL queries in slim framework?

Question

SQL

I don’t think much in php, there was a need to make a small socially useful site, I wrote everything in one file, a site of 5 pages with a database, everything works fine.
I needed to write another similar site (with the idea that it would have an API), I decided to take some simple framework, I chose slim v4, I decided to transfer an existing site to it for experiments, I found an article on Habré, I did everything according to it, though I had to exclude middleware, something did not take off, I thought that it was not needed. Templates made, everything works fine, without a database. In the v3 documentation I found "Using Eloquent with Slim", something similar to requests, but I look at this guide like a ram into the abyss.
The question is, I can’t understand where (in what place) I need to connect PDO, and where and how to call it?