I’d recommend you buy one, and do a bit of reverse engineering.

https://www.ebay.co.uk/itm/40420385...2AUMqkMsXlsyFJjtGJC8IuHw==|tkp:Bk9SR9SIs_2kYg

 

Take a look at the MAX 30101 chip, which is in stock at Mouser. It's a single chip designed for pulse-ox measurements with the required LEDs built in.

 

Here is a Wikipedia article giving basic operational details – importantly it mentions employing two leds, one red at 660nm and one infra-red at 940nm wave-length.

There must be more detailed articles available on-line – but at least the basic principle of operation is explained in the Wiki article.

https://en.wikipedia.org/wiki/Pulse_oximetry