Some are, some aren't.
Some guns like (marui), G&G, VFC, Lonex, and G&P have "marui spec" mechboxes. This means they're compatible with *most upgrades parts.
For example you have a VFC gun with a version 2 mechbox, that means you can take any properly made aftermarket parts that state they are for V2/V3 mechboxes.
Some mechboxes, like V6, require V6 specific parts.
And some guns, like ARES, SRC or KWA, require some *proprietary* parts. Like the SRC piston is slightly smaller than standard V2/V3 pistons, so you can't just buy any brand piston, it has to be SRC.
As far as manufacturers, ideally if 12 different companies make a V2/V3 part, then all 12 *should* work. However some manufacturers are worse than others, this is usually reflected in the price.
So even though angel custom parts SAY they are designed for a marui L96, I've actually seen them NOT fit. But angel custom is a pretty bottom barrel manufacturer.
Whereas if a high end manufacturer like nineball, prometheus, laylax, say that "this part is for this gun" you have some assurance that it DOES fit, and it IS a high end part.
With AEG's, many tend to be "tm spec", as having proprietary parts tends to throw many customers off. Not just because you're limited in upgrading performance, but most importantly when something BREAKS, you're limited or restricted in what parts you can use to fix it.
Parts being gun and manufacture specific is VERY common in pistols. Wherein a glock usually takes all different internals than an M92 or 1911. And where if 4 companies make a sig 226, they could use all different internal components.
So it's a good idea to steer your purchase (especially with pistols) based on their aftermarket support.
For example; VFC makes a well reviewed M&P pistol, but it has no aftermarket support. Whereas the marui M&P has quite a few aftermarket parts available.