A cover letter example is two or three requirements from the posting, each pointed at a fact already on the resume. It is not a second resume and not a new biography.
Open with the role. Then one paragraph per requirement you can prove. Close without adding a skill that the CV does not contain. If a line is not on the resume, delete it from the letter.
German employers often expect DIN 5008: sender block, date, subject, salutation, body, close. The layout is fixed. The sentences still have to come from the Lebenslauf. The German guide covers the block order.
Usually well under a page: a specific opening, two evidence paragraphs, and a close. Length does not replace a requirement you cannot prove.
Yes, when the requirements differ. Keep the facts. Change which facts you select for this posting.