After reading some recommendations on the forums, I have decided to create groups for all of my things so it is easier to replace them if need be as I will only have to replace the device in one group instead of 20-40 rules and dashboards if I use them directly.
There is an app to create groups for lights and switches, however, I couldn't find anything for the buttons and sensors (Motion, contact, Temperature sensor etc.). Am I missing something or it doesn't exist at all?
There is nothing built-in for this; groups are mostly intended for devices that you can control (and how they report back out of that can vary depending on the options you choose--e.g., what the group shows for on/off versus the individual devices, etc.; I imagine this gets even trickier for sensors).
There have been some community efforts to do something that sounds somewhat similar, "combine" multiple sensors into one via some sort of average or other value based on the individual devices. Cobra's Average All comes to mind, and I'd be shocked if BPTWorld didn't also have something, though I can't name it off the top of my head. These aren't really intended as a 1:1 mapping of a "real" to virtual device, though if the app and virtual device you're ultimately proxying these things through supports the attributes you're looking for, it may work.
Personally, I'd consider this more work to set up than it would be worth for the hopefully rare event you need to change or replace a sensor, but that's just me. 
And if you're truly replacing a device, the C-7 hub has a "Replace" feature for Z-Wave that should work if the original device is truly dead (I've had mixed luck using the hub's built-in feature for this and find it easier with a secondary controller, but either way should theoretically work). It's not quite as easy for Zigbee, but as a tradeoff, unlike Z-Wave, you can reset and re-pair a known Zigbee device if it "falls off" your network for some reason, and the hub will recognize it as the same device.
This is what I use. Works great for me.
There is a built in app for motion, Zone motion controller. This allows you to combine multiple sensors into a single zone.
Also as eluded to in above post, I use an app to combine temp sensors for average temp. I found one that Bruce posted some time back here and it works pretty well.
Download the Hubitat app
