The backlight waits for BL

A switch, not a supply

The backlight is LEDs behind the glass, inside the panel. Their anode is on 3V3. Their cathode comes back out on contact 1 of the ribbon socket, through R6, 2.2 Ω, to the drain of Q2. Q2 is a BSS138, an N-channel MOSFET: a switch that a voltage on its gate opens and closes. Its source is on GND and its gate is BL.

Between the gate and GND sits R2, 10 kΩ. With nothing on BL it holds the gate at 0 V, so Q2 is off and no current flows through the LEDs. That is deliberate: a floating gate would drift, and the backlight would flicker with whatever the pin picked up. It also means that the backlight is off until your sketch drives BL HIGH.

The page this book replaced said the backlight was "always on, or PWM-controlled". It is never on by itself.

What your pin carries

Almost nothing. A MOSFET's gate takes current only while it switches. Held HIGH at 3.3 V, BL feeds only R2: 3.3 V across 10 kΩ, 0.33 mA. The backlight's own current runs from 3V3, through the LEDs, R6 and Q2 to GND, and never through your board's pin.

So any output pin will do, and the first line of every sketch in this book sets it HIGH before anything else.

Dimmed, or always on

PWM on BL dims it. Switch BL on and off hundreds of times a second and the backlight is on for that fraction of the time: analogWrite(BL, 64) is about a quarter. On an ESP32, an ESP32-S3 and a Pico any output pin can do it. On an Uno, D7 cannot: move BL to D6 or D5.

The BL-3V3 jumper keeps it on. A blob of solder across the pads marked BL and 3V3 on the back ties the gate to 3V3: on whenever the board has power, and one wire fewer. Then BL must stay unwired, since a pin driving it LOW would short 3V3 to GND. The two jumpers has the rest.