I just got a (used) Nexus 7 (2013).... And it goes to standby too quickly too often Is there are a way to configure it, with an app or guide, to be able to READ "something really interesting" in this tablet without the screen constantly randomly going off too soon?

I would understand that the tablet had something that told the system to keep being awake while I'm using "apps" that are obviously for "reading"?
The screen doesn't randomly turn off: it turns off after a fixed time. You can change how long before the screen turns off in the main device settings: Display → Sleep. This applies regardless of what app is in the foreground.

If you like, you can use a third-party automation tool such as Tasker or Llama to change the screen timeout according to what app you're using. This question explains how.

Samsung phones also have a feature called "Smart stay". This activates the front-facing camera just before it turns the screen off, to check whether you're looking at the screen: if so, it leaves the screen on for a while longer. This isn't a standard Android feature: it was developed by Samsung and only available on their phones.