Military manufacturing refers to the part of overall production that is used to manufacture ships of all classes at shipyards.
Generation
Military manufacturing is allocated by assigning parts of your economy to manufacturing, and then setting the social/military manufacturing slider. Unlike social manufacturing, modifiers that affect only military manufacturing are extremely rare. This means that most important ways of enhancing military manufacturing are those that increase your overall manufacturing.
Planetary sponsors
One difference between Military and Social production is the fact that up to five planets are able to combine their military output at a single shipyard.
Only limit to this is the diminishing returns on long distances between the planets, or sponsors, and the shipyard. The link is at full capacity when the distance between the shipyard and sponsor is six tiles or less, and the percentage of the original military production that is available to the shipyards starts to drop as the distance increases after that.
Certain race traits and ideologies can decrease the effects of distance on the production.