There’s not much more to add to Angelos answer, except this:
Try to avoid vendors, who give you only control to your hardware via the cloud. At first, you’re depended on that cloud service. Apart from monthly/yearly fees, more and more vendors take for that Service: what if that service isn’t available - or at worst shut down? Secondly, openHAB calls itself “Intranet of Things”, which is especially in a home automation context very vital! You don’t want your home exposed to the internet without you controling it.
So, as openHAB is very broad in connecting different vendors, you won’t have to buy all your sensors, actors and stuff from only one vendor. What openHAB does is having a bridge between all those vendors and technology.
For instance, if my OneWire outdoor sensors pick up enough light and heat from my soutside of my house, my KNX controlled blinds go to defined positions. If my weather station (which updates openHAB via MQTT) detects rain/snow in temperatures near freezing my blinds go up for not have them freezed in down position…
But the important thing is, my home isn’t solely dependent on openHAB. Everything works without openHAB also. openHAB is a means for the dull tasks of automating or having a visualisation of my items to trigger them from different points.
But to your question again:
Everything in my home automation (except unfortunately my doorbell…) has either its own API, or I can have some way of accessing their data or commanding actions directly. Try to buy from vendors, who offer APIs or easy accessible other ways of communicating without having to expose it to the cloud. (If you like, you can use openHAB Cloud for use outside your home.)