As a German national, this person would be eligible for the Visa Waiver Program, a program available to citizens of 30 or so countries, mostly friendly European countries and a few in Asia (Japan, South Korea, Taiwan)–Canada is not on this list because they can enter under an even less restrictive program.
The Visa Waiver Program requires you have an ESTA, which is an electronic travel authorization for travel into the United States.
Under the VWP, you can cross from the United States for short trips into Canada, Mexico, or a few small island nations you’re considered to still be utilizing your 90 days, and it doesn’t end that 90 days early nor does it reset the 90 days.
Unlike a traditional Visa (tourist Visa B-2), if you are in the country under the VWP you cannot extend your stay, or convert to a different classification. So you cannot stay for 90, then get a tourist Visa and continue staying, you have to leave after 90 days. There is not actually a limit to the VWP usage along the lines of “you can only be in country for 90 days out of every 180 or 365 days”, you can, in fact, leave the United States and re-enter, and get another 90 days. The major exception to this is the VWP does not reset a 90 day window for the aforementioned trips to certain neighboring countries (Canada / Mexico / some nearby island countries.) However a hypothetical German, could return to Germany after 90 days and then fly back into the United States a week later under the VWP and stay for another 90 days.
Where you can get in trouble, is the Visa Waiver Program must not be used to circumvent another type of visa. So, if they see a pattern of you just “resetting” your 90 day VWP over and over, it will likely result in you being questioned at some point.
Citizens of Germany who are not interested in trying to get a work permit, have some relatively liberal visa options to stay in the United States for a pretty long time. The B-2 tourist visa has varying validity and duration of stay windows based on your country of origin and various reciprocity agreements. However, it is pretty liberal for Germany–a B-2 Visa issued to a German national will have a validity of 10 years, and an initial duration of stay of 6 months, which can typically be extended upon request. The validity window of 10 years means you can leave the United States and re-enter repeatedly during that 10 year window.
The big thing with the B-2 tourist Visa, is it cannot be used to “live long-term” in the United States and cannot be used in a way that shows such intent. It also cannot be used for extended business trips or attempts to work (there is another visa for extended but temporary business trips.) In practice if say you were a rich German who wanted to visit the U.S. and party on the beach in California for a long time while crashing with friends, you probably would not run into trouble spending many months and getting extensions on the Visa. But if you did things like rented an apartment and etc that showed signs you were intending to live in the United States on a permanent/long-term basis, they would likely conclude you were attempting to misuse the Visa and take action against you.
These sort of Visas are not tightly tracked, and the reality is many European citizens come into the U.S. on B-2 visas and stay for literally years.