Best place for the Pi-Hole

Question (short):

Where is the best place to make the most of Pi-Hole?

Many homes would have a similar network when you improve your wifi-signal. So, I'll try to talk as generic as possible.

Physical connection:

WAN --> ISP-modem (A0)
--> A1) Devices I don't control (thinking of decoder from ISP, ....)
--> A2) WiFi-router to improve WiFi-signal
--> A3) Pi-Hole

WiFi-router (B0) creates a network for all of my devices
--> B1) Devices I control (printer, pc, smart TV, IoT, ...)

Logical connection:

Devices from network B gets an IP from the WiFi-router (B0 = A3). The DNS referrers to the WiFi-router or directly to Pi-Hole (A3). Devices on network A gets an IP from the ISP-modem (A0) where the dns is referred to the Pi-Hole (A3). Pi-Hole itself referrers to internet-DNS.

Question:

How do I block ads on all the devices from both network A and B? Or should I forget network A and move Pi-Hole from network A to network B? Devices will be placed in network B unless it won't work.

Which device must refer to the Pi-Hole dns to make the most of it? Should the device itself have to point to there or can the router point to there?

im guessing your networks are vlan's?

I run a Unifi UDM-Pro with 3 networks, ALL served by 1 pihole

Vlan1 - LAN (trusted Computers/Laptops/NAS)
Vlan2 - iOT (streaming devices/cameras/phillips hue/chromecasts) etc...
Vlan3 - Guest (guest lan for friends/family)

i have the pihole on Vlan2 but with firewall rules allowing
vlan1 & 3 access to the pihole ip address UDP53
vlan1 specific ip access to the pihole for mgmt & updates (22/80/443)

all works fine the pihole serves for all 3 subnets

In my case, there isn't a vlan config present. Network A is outside network B (WAN-port of the WIFI-router).

So, if I "translate" your setup to mine, then is the best place for the Pi-Hole at A3. The devices have to point to Pi-Hole itself.

Thx