In brief, it’s predominantly junk.
Don’t know much about the history. Suspect it was originally a genuine German firm who got bought out in an attempt to add some credibility to crappy tools for the European market. It could be one of those made up brand names though!
You tend to find them at places like DIY shops who don’t offer an in-house brand, but want to offer something equally crappy.
In the U.K. you find them at Screwfix, which isn’t a good start as they’re not particularly cheap, but rather sell cheap stuff, which is different. Screwfix have their own brand, Titan. I actually have a big Titan hammer drill I got for a project. It was so cheap I treated it as disposable, but I’ve got to be honest it’s not a bad tool. Einhell seem to be a notch down from even the Titan.
I once used an Einhell angle grinder, and it was, without doubt, the crappiest and most dangerous tool I have ever used! I drove 12 miles to collect my own, then chucked the Einhell to protect future users...!