Genuine part generally means better quality but not always. There is also a lot of cases where maker X sells a part but they are actually the original maker. It wont come in a LR bag but will be exactly the same part. Unfortunately there are also very poor quality pattern parts that actually do more harm than good. Chinese oil pump gears from a few years ago being one of the well known ones. The gear sheered of the end of the shaft leaving a damaged cover and no oil pressure. As for your pressure sender I would doubt there is such a thing as a genuine part any more given its age. Smiths do still exist by the way but they are under another company now. I don't know the dash of your car but if they are individual gauges then maybe consider fitting a mechanical one in its place.
Generally parts suppliers 'own brand' stuff will be the chinese parts. Some aftermarket stuff like Hengst filters, sachs clutches, Valeo Headlamps are oe fitment, i.e. these are as marki says the same parts used by the manufacturers (or 'vehicle assemblers' as the parts industry like to call them). Good parts like these will not be as cheap as chinese made stuff obviously but will be a whole lot cheaper than paying retail prices at the main dealer, while being identical in fitment.
Caerbont Automotive (CAI) which is an amalgamation of Smiths and VDO. Gauges are still sold under the Smiths name tho. There also seems to be some tie with Telemetrix https://www.europaspares.com/INSTRUMENTS/ . I think your problem is that your instrument is calibrated to some specific sender that seems to be made of unobtanium going by the price. I take it its not an option to replace the guage, even just the working parts leaving the face as stock? Another option would be to get it re-calibrated so it can use a general sender.