This quick blurp was brought on by an article at Windows Central that I believe got it a bit wrong. In the days of Windows XP & 7 there weren't an awful lot of low powered Windows devices -- common practice was to run the equivalent of today's Intel i5 & i7 stand-alone CPUs [vs. SOCs or System On a Chip etc.]. And at that time for decent performance out of your PC you ran 4GB RAM in XP, while 7 64 bit very much liked 8GB.
Today we have low powered SOCs, lower powered mobile versions of main stream stand-alone CPUs, more of the lower end CPUs that used to really be frowned on, e.g. the Celeron, faster hard drives with faster disk access, plus SSDs, together with Windows 10, which is much more forgiving of low end devices. If you run a stand-alone i5 or i7 [or AMD equivalent] it's hard to go wrong with the same 8GB of RAM that let the CPU perform the way it's supposed to in 7. You can get away with 4GB, especially with the i3 & lower CPUs, while 2GB is the new minimum for something like a Celeron or Atom in mobile form or SOC. Hard core gamers, those working with media files, & many of those running VMs can run 16GB [or more], but then things get surprisingly iffy.
The way it works is Windows reads what it needs to from the files stored on the hard drive, & stores that in RAM. Programs you run do the same thing. When there's a danger that the device is going to run out of available RAM, some of what's stored there is written back to the hard drive, which is only a problem in so far as reading & writing to the hard drive is slower than reading & writing to RAM. Windows 10 adds a bit of complexity, compressing some of that data stored in RAM, whether you need it to or not. So, too little RAM for what you're doing & the device spends too much time reading & writing to disk, which slows things down -- any excess RAM, i.e.more than what's needed, just sits there unused. Put another way, more RAM will only help if you've got too little right now for what you use your Windows device for.
There's one caveat... you normally have to have a 64 bit copy of Windows to take advantage of more RAM. Whether your device can run 64 bit Windows or not is dependent on the device's bios. A 32 bit CPU [assuming you can still find one] will have a 32 bit bios, but so will some devices with a 64 bit CPU.
If you build your own or add to an existing PC/laptop, RAM compatibility is also a bit of a crap shoot. The safest way would be to stick with brands & models that the manufacturer of your device tested & certified. The problem with that is that very often if you can find that info, it doesn't list any of the brands & models you'll find in stores [B&M or online]. Your best bet then is user reports, e.g. in forums -- you might find that some brands like Asus just prefer some brands of RAM in general -- & then make sure you can return it if need be.
The fastest use or access of RAM happens when you have & use dual channels. You'll find however that hardware can be even pickier about which RAM it works with when you use dual channels. Going above 8GB of RAM, usually 16 or 32GB, hardware can become pickier yet, & in fact you'll find some hardware that apparently will not run 16GB or more, despite being labeled to do so. When you do want to run more than one stick of RAM, ideally they'll be part of a matched set -- you want to match not just brand & model, but lot# or production run.