How is a BME280 breakout wired to an ESP32 over I2C, and why are the sensor's pins labelled SCK and SDI rather than SCL and SDA?

Four wires: VIN to 3V3, GND to GND, SCK to IO22 (SCL, the clock) and SDI to IO21 (SDA, the data). The chip also speaks SPI, and SCK/SDI are its SPI pin names, which double as the I2C clock and data pins.

* Two of the four wires carry power, two carry the bus. *

Three details make this wiring work:

• The pin names. The BME280 supports both SPI and I2C. Its pins are named for SPI (SCK = serial clock, SDI = serial data in), and in I2C mode the same pins serve as SCL and SDA. The SDO and CS pins stay unconnected for I2C.
• 3.3 V, not 5 V. The ESP32's I/O runs at 3.3 V and its pins are not 5 V tolerant. Many breakouts (Adafruit's among them) have a regulator and level shifters, so they would accept 5 V. Cheap modules often do not, and the bare chip is a 3.3 V part. Powering from 3V3 is right for both.
• IO21 and IO22. These are the default I2C pins of the Arduino core on a generic ESP32. The ESP32 can route I2C to almost any free GPIO, so they are a convention you can change in code, not a hardware requirement.

Go deeper:

• Adafruit — BME280 breakout pinouts — which pin does what in SPI and in I2C mode, and the address jumper.
• Arduino-ESP32 — I2C API — the default SDA/SCL pins and setPins() to move them.
• Wikipedia — I²C — the two-wire bus, open-drain lines and addressing.