Most of the older Polar HR sensors didn't use cloud services but they typical worked with an auxiliary chest strap (it wasn't just wrist strap), they were mostl

Most of the older Polar HR sensors didn't use cloud services but they typical worked with an auxiliary chest strap (it wasn't just wrist strap), they were mostly for monitoring your performance during physical activity not necessarily for the 24H wellness type of use-case that people usually look for these days. If you are into DIY, you don't necessarily need to be a DSP guru, there already quite a few 'good enough' IC's, like the MAX30102 (Maxim) for HR and oximetry, it doesnt need any major analogue considerations you just need to implement the communication side (I2C)to extract data.