Flash the OS

Flash raspbian light 32bit on your raspberry pi zeros

Upgrade packages

sudo apt update && sudo apt upgrade

Install pip3 and flask (only required on the server)

sudo apt-get install python3-pip
pip3 install flask

Enable remote gpio (on all clients)

sudo raspi-config

install pigpio (on all clients)

sudo apt install pigpio

enable the pigpio service (on all clients)

sudo systemctl enable pigpiod.service
sudo systemctl start pigpiod.service