Especially if you are going from legacy to UEFI.
If that's the case, be aware you need to convert the disk from MBR to GPT manually. The Windows installer can't do this for some odd reason.
I'm doing a clean install on a laptop which currently has win 7 on it, it obviously has a drive partition 1 (system reserved) and partition 2. Should I delete these and format before installing?
If you have win 7 you can upgrade to win 10 for free and then do a clean install
cause the upgrade version will not work right,
Don't forget that if you don't like the start menu you can always add classic shell and customize it to your liking.
Calling everything an app is confusing me though, is that their term for all programs now or is an app still something designed for the mobile platforms?
That wasn't my experience when I did it. I just had to delete all the partitions on my drive and the installer was able to convert without issues.
I know that the 7 installer can do it from a clean state (after Diskpart "clean"). You don't have to convert mbr/convert gpt in that case as it will choose the appropriate one.