There are two methods:

• You can use caps lock for the capitalized umlauts and caps lock and shift for the capitalized French accented vowels
• You can use the accent buttons and combine with a normal capitalized vowel. For example, the button between ü and enter is the two dots button ¨, so you press two dots, then shift-o and get a capital Ö. Same for the French accented vowels the two buttons on the left of backspace have ´ and ` (with alt-gr and shift respectively) and you can combine those with shift-e for É È.

The second method sounds convoluted, but you get used to combining keys anyway. For example for the circumflex ^ because â ê î ô û don’t exist pre-combined on this keyboard layout. The same goes for some rarer combinations like ï, which despite the dots isn’t a German umlaut, it’s an i with trema for use in French for example in haïr, to hate.

German only really introduced capitalized umlauts for printing around 1900, so people used to use the combinations of the vowel with e for capitalized umlauts in print. Then the first mechanical typewriters again didn’t all have umlauts, or sometimes had only small umlauts. The combinations with e is also used for systems that have technical limitations. If they are ASCII based for example. Therefore even today people are somewhat used to it, so if you were to write Oeffnungszeit instead of Öffnungszeit nobody would bat an eye.