When you have a wire-connected computer with a unused wireless interface, an option is to use the device as an access point to improve the coverage of your WIFI network. This can be done by setting up hostapd, which takes care of wireless interface configuration, authentication and security. However, in order for the wireless traffic to actually go through the wired connection and onto the internet and vice versa, the two interfaces needed to be bridged somehow. There are several scenario for configuring the bridge connection. For the NetworkManager, this can be down with the following.

nmcli con del <existing connection for eth0>
nmcli con add ifname br0 type bridge con-name lan
nmcli con add ifname eth0 type bridge-slave master lan con-name eth0-lan

Afterwards, hostapd can be started with the following configuration:

interface=wlan0
bridge=br0
ctrl_interface=/run/hostapd
ctrl_interface_group=0
ssid=test
channel=11
wpa=2
wpa_passphrase=Hello123
wpa_key_mgmt=WPA-PSK

However, the order of two is important. The connections of NetworkManager need to be configured before the hostapd is started. Otherwise, the bridge interface will be taken over by hostapd before NetworkManager can configure it properly. It is less than straightforward to enforce such an order with systemd. For current Debian unstable, the dependency in /lib/systemd/system/hostapd.service need to be corrected from After=network.target to After=network-online.target .
Alternatively, you can create a file /etc/systemd/system/hostapd.service.d/netwait.conf with:

[Unit]
After=network-online.target

to change the dependency.