Hardware needed for bootloader

You need at least an UART (like this one: https://amzn.eu/d/6NM1mPW ) and then something to connect the pcb tag with the uart.

You can buy the fitting tag connector ( here are listed some: Debug port | Toniebox Hacking )

Or

Use thin wires like shown here: 0.35mm resistor wires as Tag Connect alternative - TC2050