Setting up a Raspberry Pi as a Wireless Access Point ( gut erklärt ! )
https://www.raspberrypi.org/documentation/configuration/wireless/access-point.md

Access point as WiFi router/repeater with additional WiFi-dongle
https://raspberrypi.stackexchange.com/questions/95072/access-point-as-wifi-router-repeater-with-additional-wifi-dongle

RPI3 Raspbian Stretch regular connection on wlan0 AP on wlan1
https://raspberrypi.stackexchange.com/questions/88599/rpi3-raspbian-stretch-regular-connection-on-wlan0-ap-on-wlan1

Howto migrate from networking to systemd-networkd with dynamic failover
https://raspberrypi.stackexchange.com/questions/78787/howto-migrate-from-networking-to-systemd-networkd-with-dynamic-failover/78788#78788