Viewing NEF files only

To filter your view (of any folder or collection) by filetype, it is helpful to save a named filter preset from the filter bar, as has been suggested, or else to use LR's builtin ability to show Raw images vs non-Raw images. But what this will not do - because it cannot - is: distinguish between those JPGs which are acting as imported originals you are editing FROM (straight-out-of-camera JPG, chiefly) - and those which represent exported outputs, with your past editing irretrievably written in (also, downsized and cropped etc to suit some particular usage).

The reason this matters is that these two classes of JPG serve two very different purposes, and only one class is technically suited for further modification. So the other class - the exports - is more of a hindrance than a help within the LR Catalog. That doesn't mean these exported JPGs need to be deleted from disk. Only, in the Export dialog going forward, the option to re-import can be un-checked. And those already imported can be Removed from Catalog safely (without deleting from disk). That requires a reliable way to distinguish these - but it sounds as if a folder system has been used already to do that, which is ideal. So those folders (named "small", or whatever) and their contents might be removed easily. You can still access all that in the normal way with a file browser, outside of LR.

LR then doesn't manage those exports at all. Whatever EXTERNAL usage called for making them in the first place, can access them directly where they are saved on disk.

Inside LR you have got the "live" edited Raw to call on, which contains the full picure information at high bit depth, with your up to date editing applied dynamically, and that is the best available basis for anything further you may want to do. If you want to have the option to revert to a particular editing state that matches a certain export moment, the Export appears as a dated step in the History. Also you can make a virtual copy or save a Snapshot in order to preserve that as-exported editing as a separate entity. But this will itself refer, nondestructively, back to (the same) Raw original as your latest editing does; and not to some destructively processed JPG original; with all the practical advantages which working from Raw entails.

Often a physical book library has a photocopier that you can use. But no resulting photocopies get put back in the library shelves; only the book itself lives there. You can think of LR exports like those photocopies - derivative, partial, ephemeral.

You may consider something else: Don't save all those versions back into your catalog in the first place! The whole idea behind Lightroom is that you do not have to do this, because you can export a jpeg copy any time you need it with the specific settings you need for that specific purpose. That means there is no reason to keep all those jpegs after you've exported them and used them.

If you only shoot with Nikon, then you can also use the 'Metadata' filter and filter on 'File Type' = 'Raw'. All your raw files will be NEF files. Maybe you did that on your old computer.

Either that or just use the filter bar. You can make a filter bar preset for it, so you can select it in the right corner (where it now says 'Custom...').

Why not go to the collections panel and click the + icon and choose to create a new smart collection based on filename. See image below.