I would be interested in VOC too. This manual for the hardwired heat alarm has info on placement.
https://digitalassets.resideo.com/damroot/Original/10000/M08-0026-008.pdf
For any battery operated ones I imagine placement would be similar. This product is not hardwired, does require a base station though. The base station pairs with other X-sense devices. So this could be a possibility for homes without hardwiring. There is an app to get notifications on as well.
https://www.x-sense.com/products/x-sense-smart-heat-alarm-xh02-m-with-10-year-sealed-battery