[NAME] Antonio Rose  HDMI 1.4 supports up to 2560x1440 (aka 1440p) 60 Hz or 4K 30 Hz.  Even assuming your display would use the standard 60 Hz refresh rate rather than being a high refresh rate display, 27" displays are available in a variety of resolutions, including 1080p, 1440p, 4K, and 5K.  If your particular 27" display uses either of the first two resolutions and has an HDMI input, then I would recommend using a regular HDMI cable to connect the HDMI 1.4 output on your system to the HDMI input on the display.  You could use USB-C/Thunderbolt if you really wanted to, but given that that port can be used for a variety of other things, I think it makes sense to keep that port available and instead use HDMI, which can only be used for video output anyway.If you have a 4K or 5K display, you would need to use that USB-C/Thunderbolt port, either with a USB-C to DisplayPort cable, a USB-C to HDMI 2.0 cable, or an actual Thunderbolt 3 cable, depending on the particular display.If you'd still like more assistance, it would be helpful for you to specify the actual make and model of the display you're working with so that its resolution, refresh rate, and available inputs are clear.And yes you can copy/paste across displays because you're essentially just giving your system more workspace.  But it's still the same system, so you can drag application windows back and forth and even span applications across display boundaries if you want.  The applications on each display aren't isolated from each other.  Your system will still work the same as it always does.  You'll just be able to see more content at the same time.
I recently got an Inspiron 15-5510 and I'd like to add my former desktop's 27" Dell monitor to it for use when I want a bigger screen. I don't plan to use it for watching video, just for when I need more space, such as when I have a wide spreadsheet and need a split screen. I don't know anything about adding an external monitor. Is getting an adapter for the HDMI 1.4 adequate, or is getting one for the Thunderbolt pretty much the same but much more functional? Or does using the Thunderbolt require a docking station, which costs a lot more?  Also, once I hook it up, can I cut and paste from the external monitor to the laptop monitor? 
HDMI 1.4 or Thunderbolt?