I know that you can create a WiFi access point with Raspberry Pi but being connected via ethernet to an existing network. How can I create my own independent WiFi network fully generated by RPi? if you would be so kind to give me step by step
You can use this as a guide:
I had all kinds of issues and everything is solved, NOTE: read my own answer if using newest raspbian and rpi3
An access point, like a router, maintains a (W)LAN of it's own. Also like a router, when the uplink goes down the LAN the AP provides remains active. Point being, it doesn't have to have an uplink to start with if all you want to do is connect the devices that connect to it (or just connect another device to it without the internet). If you have a phone with a hotspot, that's an access point, and it will work when the phone is offline. This is exactly the same thing.
There are some good Q&A's here on the topic of APs and hotspots, most of them step-by-step self-answers by Ingo: