Right, they are "end of speed limit". So, if there's speed limit signs you'll find another speed limit or this "end of limits" sign afterwards. This doesn't mean "go qs fast as you want", it just means there's no additional limit on top of what the road type says.

So if it's a road where ongoing and incoming traffic is just divided by road markings, the limit is 100 (or below if there are speed limit signs), if the road has two halves with some barrier in the middle ("baulich getrennte Fahrbahnen") and at least two lanes per direction the recommended speed is 130 with no hard limits. (But the insurance might get back to you if you were driving tooo fast)

Btw interestingly while we may drive really fast we have a law against racing your car (including racing to the next stop light), if you do that you might lose your driving license and not get it back for a looong time. (And that's really expensive)

Last, if you do traffic violations which are more than a misdemeanor (for example tailgating) and they catch you, your license will be suspended for at least a month, but typically more.

Interestingly minor violations like being up to 20kph too fast outside of city limits is. among the cheapest all over Europe. But do something bad and you'll ride a bike for a few months.

Edit: thank you autocorrect for correcting so if to son of.... now everyone knows what a filthy mouth I am.

This is the sign, hard to miss:

Usually I only see them at the beginning of no limit sections. Google is more often wrong than the dashboard info in my car that is reading the signs.

And sorry if I misread you, but based your questions you seem quite anxious about the autobahn. Maybe this is just your general careful personality, or maybe you’re young and not an experienced driver already. Regardless, I urge you to be careful about very high speeds, it is really a very different world and can be dangerous. You need to pay an insane amount of attention and react very quickly and confidently to the intentions and actions other cars.

Again, sorry if I misread you based on your comments, but if you don’t feel confident enough yet, try getting some more driving experience at regular highway speeds, maybe even test yourself on a closed racing track, or do some driving training.