Regarding wired protocols

[pillarist]

I am using C with ESP32 and MicroPython with the Pico. I want to configure the Pico to work as an SPI slave, but I couldn’t find a way to achieve this. Is there any possibility of making the Pico function as a slave using SPI, I2C, or any other protocol?

[shariltumin]

You can have a try with UART.

[peterhinch]

As @shariltumin says, the easy solution is a UART. Alternatively an SPI slave could be designed using the PIO linked to a DMA channel. The very beginnings of such a solution is here but this only needs to read a single byte.