High end Samsung phones and some Garmin watches have pulseox sensors too. They're not super accurate, but better than nothing. Some Apple phones have them too, but they aren't accessible unless your phone is jailbroken and you have an app to get to them.
"Silent pneumonia" - the cause of most COVID-10 deaths.
Get yourself a pulse oximeter. These are some good, cheap ones: https://reviews.chicagotribune.com/reviews/best-pulse-oximeters
High end Samsung phones and some Garmin watches have pulseox sensors too. They're not super accurate, but better than nothing. Some Apple phones have them too, but they aren't accessible unless your phone is jailbroken and you have an app to get to them.