SPI (Serial Peripheral Interface) uses a clock, a data line from controller to peripheral, a data line back, and one chip-select line for each device on the bus. Because data flows both ways at once and there is no addressing overhead, it is much faster than I2C and is the usual choice for external flash memory, SD cards, displays, Ethernet and radio transceivers such as LoRa chips, and high-rate sensors.
The cost is pins and the lack of a formal standard. Each additional peripheral needs its own select line, and clock polarity and phase settings vary between chips, so every pairing has to be checked against the datasheet. In practice most IoT devices use both: SPI for the one or two parts that need bandwidth and I2C for the collection of slow sensors.
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