Vehicle manufacturers make very little themselves, mostly only body panels, engines and gearboxes. Although many do not make their engines, and automatic gearboxes tend mostly to be specialist sourced.
Many of the OE suppliers also supply other manufacturers, and will often use the range as the basis for an aftermarket presence, selling the same stuff in thier branded boxes.
Then, to make the range wide and complete, they buy-in from other similar OE suppliers. These OE quality suppliers generally only source and sell similar quality components, not wishing to dilute their brand with inadequate parts. The major brands have agreements and share to ensure they all have a competitive offering.
The highest demand parts will often have several manufacturers, such as oil filters for the FIRE engine. Lower demand parts may well only have one manufacturing source, despite being available from every supplier.
The risk of sub-standard parts mostly only exists with the highest demand parts, as they are the only ones with enough volume to justify the cost of tooling.
With many components a vehicle manufacturer will source from two or more suppliers, so an OE clutch for your Panda may have come from Valeo or LUK. Most good aftermarket brands will be one of those in their box.
While a vehicle is in production, volumes are great, so replacement parts will be the same as that fitted on production. The component supplier makes a few more than necessary for the production line, for the replacement part stock.
Once out of production, component volumes shrink. Many component manufacturers cannot make small volumes economically, so replacement part manufacture is passed to another company. Often this includes the original specifications and drawings being transferred. Once a car ends production, OE branded parts may be exactly OE, to OE spec manufactured on OE tooling but by a different manufacturer, to OE spec on new tooling from a different supplier, or a good branded aftermarket copy, sanctioned by the manufacturer. Sometimes an aftermarket manufacurer will already have tooled up for a component, then may modify its tooling when they get the OE repalcement contract.
Now teh hornets' nest is really buzzing, and anyone reading this far is hoping it won't go on much longer - sorry.
For any replacement parts, any of the good brand names should be fine. If in doubt, ask on here. Reputable sources claiming reputable brands are trustworthy, but be wary of other sources. Ebay can be a nightmare. Some of the traders are sub-divisions of the major aftermarket suppliers, some are dealers, some are honest people finding a way to make a living without needing retail premises. Some are rogues. Cheap, poorly made, unsafe, etc., find a route to market via the easiest path, so more likely via Ebay than Euro/GSF etc. Although some of the cheaper offerings from factors can be at the low end of adequate.
There are not really that many oil blenders, so the aftermarket cheaper brands come from them, blended to the specs required. An oil supplier cannot risk claims if thier product fails, so any decent brand name should be trustworthy. Supermarkets have a reputation to protect, so even if oil is a small piece of their overall offerring, they cannot risk problems. They will source it from one of the blenders, but likely leave it to the supplier to recommend which specs to supply. Some may even come from the big names, but finding that out might be a challenge.