VirtualWire Library, for very cheap wireless communication
Nettigo: nRF24L01+ compatible module - wireless communication for Arduino
Wireless Communication Between Two Arduinos - Circuit Basics
Wireless communication between two Arduinos using inexpensive RF modules | Embedded Lab
Amazon.com: HiLetgo ESP32 SX1262 LoRa Kit 863-928MHz Bluetooth 4.2+WiFi+LoRa LoRaWAN Wireless Communication Development Board with 0.49" OLED Display 64M Bits Flash Works with Arduino IDE Type C : Electronics
Wireless Communication: Interfacing 433MHz Transmitter and Receiver Mo – QuartzComponents
How 433MHz RF Module Works & Interfacing With Arduino – Robocraze
Wireless communication in IoT systems using Arduino MKR modules | Electronic components. Distributor, online shop – Transfer Multisort Elektronik