I²C bus basics
Two wires, many devices, each with an address.
I²C uses SDA (data) and SCL (clock), both pulled up to the logic voltage. Every device has a 7-bit address — LCD backpacks are usually 0x27 or 0x3F, SSD1306 OLEDs 0x3C.
A 5 V I²C module on an ESP32 can pull SDA/SCL up to 5 V. Use a level shifter or a module that runs at 3.3 V.