! ***   WLAN Repeater – Wie baue ich mir meinen eigenen WLAN Repeater mit dem Raspberry Pi? deutsch
ohne zusätzlichen WLAN USB-Adapter
https://lookslikematrix.de/raspberry-pi/2019/07/15/wlan-repeater.html

***   Access point as WiFi router/repeater, optional with bridge
engl.: Base von lookslikematrix + Erklärung
The Raspberry Pi is able to work as a wireless hotspot and simultanous 
connect to another hotspot as client. But the wifi device on the RasPi 
has a general limitation. The client connection cannot be bridged because 
lack of WDS (wireless distribution system) that is needed for bridging on wireless.
https://raspberrypi.stackexchange.com/questions/89803/access-point-as-wifi-router-repeater-optional-with-bridge
https://raspberrypi.stackexchange.com/questions/89803/access-point-as-wifi-router-repeater-optional-with-bridge/89804#89804

***   Verlängerung der Reichweite eines WLAN mit Hilfe von portablen Raspberry Pi als Brotkrumen
(Bachelorarbeit)
https://www.christianbaun.de/Abschlussarbeiten/Bachelorarbeit_Tim_Scherffig_2019.pdf

***   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

***    using a second wifi dongle is really very easy. 
With systemd-networkd and wpa_supplicant we have everything on the Raspberry Pi to setup 
what you want in a consistent way. There is no need to install additional software and 
fiddle with hostapd and dnsmasq. You have to switch to systemd-networkd and then simply 
set up wpa_supplicant one time for wlan0 as client and one time for wlan1 as access point. 
Then configure the interfaces and it's done. Let's do it. For reference I use Raspbian 
Stretch Lite 2018-11-13 updated with sudo apt update && sudo apt full-upgrade && sudo reboot 
done at 2019-02-02.
https://raspberrypi.stackexchange.com/questions/88599/rpi3-raspbian-stretch-regular-connection-on-wlan0-ap-on-wlan1