It's apparently doable optically, similarly to blood oxygen, albeit much more challenging. There's an actual product out for it from one of Apple's suppliers, even: https://appleinsider.com/articles/21/07/14/apple-supplier-la...

Not necessarily. See section on non-invasive methods: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7731259/

Believe they acquired a company that did it via light, hence the belief that it is coming.