Wifi AP on Raspberry Pi 4

I am thinking how it could be possible to set up the rpi4 as a wifi access point to serve all my wifi devices more or less independently from the rest of my wifi network or at least to have kind of a backup network for the case that the usual network is down.
I have many tasmota devices connected to home assistant and those could be configured to use a second wifi in case the normal network is out of order.
This could make the setup more reliable.