European tool manufacturers “trade parts” and do “OEM Manufacturing” for each other on a routine basis.
There also seem to be separate manufacturers that make certain parts for a bunch of different brands.
The angle grinder side handles usually use one of maybe three thread patterns, so a bunch of manufacturers may use the same side handles.
The only handles that are likely to be proprietary, are ones with highly visible branding, or special features like vibration reduction, or built in flange wrenches.
As for the switch, most power tool manufacturers buy components like those from specialty suppliers.
Marquardt is a major European supplier.
Bosch has sourced power tools from other suppliers at times, (at one point some larger grinders were made in Russia), and was likely the OEM supplier at others.
It’s possible that Bosch has made grinders for Metabo, or Metabo for Bosch.