Reflexive doesn't necessarily mean losing the oxygenation information, but often the devices don't care for it - might be harder than with transmissive.
You get a photoplethysmography signal in any (healthy) case when you shine any reddish light on skin and pick up what comes back or goes thru.
The bobbling up and down of your photodiode current due to blood pressure in the watched tissue, with a low-frequency change overlaid given by breathing, and if you are unlucky lots of movement artifacts if your person under test is moving... or is a hyper-neurotic cat O0 That is your raw pulse signal, just compare to a treshold,
Using at least 2 or depending on manufacturer up to 7 wavelengths, using the "color change" of hemoglobin depending on its oxygen saturation, is what gives you the SpO2 reading. (EMS Oxymetersm also the % of Hemoglobin spoiled by toxic gases)