Can I take the standalone exe from say NOX and use it on a win98 se machine and it'll work or are the installers only for current os's?

No, they are not compatible with Windows 98 or earlier; they require a minimum of Windows 2000/XP. Newer installers may not even support Windows XP/2000.

"Works on: Windows (XP, Vista, 7, 8)"

You can reasonably expect games to work on newer versions of Windows (8.1, 10, 11), but not older ones. The point of the store is to make classics playable on modern operating systems, not the ones at the original time of release.

GOG's installers indeed are not meant to be run on very old operating systems, but the games themselves may very well be still able run on the original hardware if making them run on more recent versions of Windows wasn't achieved by using any alternative engines or applying any major compatibility tweaks that break backwards compatibility, but you may need to know how to locate, export and import a few registry keys to overcome the fact that you had to install the game on a different system than what you want to use to play it.

DOS games are often installed with DOSBox, so in such cases you should be able to run them even on native DOS, at least there is no CD images you would need to burn to old enough media that the original hardware can read it.

Unfortunately GOG doesn't seem to be interested to spend any extra effort to dump all DOS games without CDA-tracks to disk, even Ultima Underworld I & II had their collection disc saved as two identical images, despite both games lacking any disc checks that would have had to dealt with in order to run them without the CD...