There are three RF interfaces commonly used for these chest strap heart rate monitors:
Bluetooth (normally BLE) - not very common but there are some out there - tend to be relatively expensive. There's a Bluetooth profile for these devices - check out the Bluetooth standards.
ANT+ - a radio standard targetted at sports/exercise equipment in general. There's an industry association at
https://www.thisisant.com/ and openly available documentation. Nordic Radio make a specific chipset for this and I suspect that is what is used by 99% of designs out there.
5kHz RF/induction standard. This is simply the actual detected heart pulses (R wave) squared up and modulated onto a 5kHz or 5.6kHz carrier. Easy to interface to with just a pickup loop, amplifier and a PLL based tone decoder like the LM567 or similar. Originally used by Polar, common on the cheapest 'no name' brands. There is a later variant with a coding burst as well, that allows you to differentiate between a small set of heart rate monitors operating close to each other.
I built a receiver for the last standard using an old transistor radio rod antenna as the pickup coil, an LM358 as amplifier and an LM567 - specifically to link to an Arduino. Worked fine with no real design effort - essentially I worked directly off the LM567 datasheet and breadboarded it as I designed it in my head/on a scrap of paper. It really is that simple, so I've not got an actual schematic to offer.
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