I can not answer you fully for the german but I can say facts from both france and German reserves (very similar).Contrary to reserve in UK or US (guard), France and german reserve are made with draftees who have completed their military service so between 10 months and 16 months depending qualifications and which can be redraft in France up to 32 years old.They were before normal soldiers.They are used to complete active units or additional units up to division (territorials or combat division).In war time they were supposed to more than double army size (France 15 active division to 29 in war time in cold war).Their officers are recently out of duty officers, or conscript reserve officers or active officers and warrant officers moved in war time to the reserve unit which are for combats units often the sister unit of an active unit.Army propose you to belong to reserve as a volonteer basis (but some units will be also filled with non volonteers in war time starting with the most recent draftee of parent units) and follow regular training after the years following military service (for officers and warrant officers mainly and you are not doing that for money as the money you receive is ridiculous).In cold war some time a unit up to division was raised to participate to manoeuver. It seems to be different of US guard system which seems a separate corps where soldiers and officers are not always formers of active units. Since the cold war army said to most of us that we were not really needed and cancelled most of training programs in France to save money and I ve heard it is the same in germany or Swiss or other European countries with conscript armies.

German Reserves are 400.000 at the moment but will be downsized so that Germany will have 505.000 Man in case of War. In the moment it would be 680.000.