FotS

MMBasic natively supports sending and receiving signals to support IR Remotes for the Sony protocol with the MMBasic IR command. It can also receive NEC codes but there is no corresponding NECsend command. TBS member @phil99 has added support for NEC and several other protocols using MMBasic. See the threads below.

IR Remote Control programs for the NEC Protocol and Variants (Samsung and JVC)

IR Remote Control Rx and Tx for Philips RC-5/6 Protocols

Link to the note below that touches on frequency and level shifting

Part of the issue of the old Sony IR receivers poor performance is their maximum sensitivity is at 40kHz, not the more common 38kHz. (Philips use 36kHz). Regarding level shifting a 5V sensor, they usually have an open collector output with an internal pullup (40kΩ is common) so all that is needed is a suitable pulldown resistor. Finding the value of the pulldown is easy as the output is idle high. 1) Before connecting to the Pico simply put a 82k from the output to ground and measure the voltage. If it is a bit high or low change the resistor to get it in the range 2.5 - 3.3V. 2) Or just add a Schottky diode from the output to 3.3V.