Well, I had a German car (diesel engine) for 12 years and changed oil according to the board computer, it came out roughly on average around 32,000 km, i.e. a little bit more than 1 year (sold it with 300,000 km after 12 years).

But the oil of a manual transmission (and as well that of an automatic one) is normally separated from the engine oil, do I read it correctly that you had a car with those in common?

All cars have one or more magnets (for ferrous shavings)it is perfectly normal, but of course they are pretty much ineffective for other metals, and more generally other possible contaminants.