What does the iptables related operation in step 11 and 7 of this tutorial do?

Question

this tutorial:

https://calap.co/blog/how-to-use-raspberry-pi-as-a-vpn-access-point--2018-10-27.html

shows how to turn an raspberry pi into a VPN hotspot.

I find the iptable configs at step 7 and 11 confusing, in that:

1. why was things done in step 7 undone in 11? To reroute the traffic through openVPN?

2. Can you please tell me what these 3 lines do? my guess:

   sudo iptables -t nat -A POSTROUTING -o tun0 -j MASQUERADE

Seems to be telling the OS to do something after traffic came out of tun0, which is the VPN process I suppose.

sudo iptables -A FORWARD -i tun0 -o wlan0 -m state --state RELATED,ESTABLISHED -j ACCEPT

Tell the OS to accept any traffic coming from tun0 and forward it to wlan 0

sudo iptables -A FORWARD -i wlan0 -o tun0 -j ACCEPT

Tell the OS to accept any traffic coming from wlan0 and forward it to tun0.

3. Theoretically the traffic should be flowing from wlan0 to tun0 then eth0 (I could be totally, totally wrong), but I saw the part where it addressed wlan0 to tun0 but not tun0 to eth0? Or is it not necessary as eth0 is seen as the default ethernet/internet outlet interface?

Sorry for the imposition brought about by such a complicated and comprehensive question!