There should be no confusion, this is all very simple.
If you have a PC that came with Windows 7 or 8, you can install Windows 10 for free.
Download the media creation tool directly from Microsoft and create the appropriate ISO image to burn to DVD or bootable USB flash drive.
Download Windows 10
Boot off the DVD/USB, when prompted for the serial key, enter the serial key on the COA sticker on your PC. If you don't have a COA sticker, the activation is integrated and you can hit the skip button. Windows should then automatically activate at the end of the installation as long as you are connected to the internet.
You are entitled to the same sku as your previous Windows install. Windows 7 Home Basic, Home and Windows 8 Home get upgraded to Windows 10 Home. Windows 7 Pro, Windows 7 Ultimate and Windows 8 Pro get upgraded to Windows 10 Pro. You do not have to do an in place upgrade, you don't even have to install it on the same hard drive.
I did over a dozen Windows 7 -> Windows 10 upgrades this way, this week at work and over 50 in total thus far. Since Windows 10 is more resource heavy then 7, I pull the HDDs and replace them with SSDs and upgrade them to at least 8 GB of RAM. All machines run beautifully and machines have clean installs. Nothing tricky about it. I can do roughly 1 PC an hour going from Windows 7 on a HDD to Windows 10 on an SSD with all company customizations made and software installed ready to be handed back to the end user. What makes it quicker now is 1903 comes with a lot less bloatware to be uninstalled.
Also did this with my 3 computers at home. I have a 12 year old HP that came with Vista with an Intel Core 2 Quad and maxed out a 8GB of RAM and a Intel SSD, got an older NVidia Quadro Pro card from a junk PC at work and the thing runs amazing for it's age with WIN10. My laptop, the computer I'm using now, is from 2011 came with Windows 7 and 16 GB of DDR3. Swapped the HDD for an SSD and put WIN10 on it the day it was released. No issues. My newest PC is from 2013, fourth generation i7 Extreme hexacore, 32 GB of RAM, GeForce 770. Configured it with an SSD from the the factory, replaced it with a larger SSD when WIN 10 came out. Again no problems whatsoever.
With the release of 1903 last week, I did fresh installs on my 3 home PCs and work laptop last Saturday. I refuse to upgrade to the latest release, so I do complete reformats every Spring and Fall. All in all I've probably done 100 - 150 Windows 10 installations and not a single hiccup.