Types of unit production:
1.At building when exact amount of resources is available.
Unit can be queued only if all units cost is available now.
Resources are reserved for all units, not available for spending.

2.Queued at building until there are enough resources.
Resources are consumed only when full price amount is available,but can be queued at advance.
Resources are only reserved for currently built unit.

3.In production as soon as command is issues, but stall when resources are scarce: this is the most complex method.
Resources are consumed per each percent of unit train time, when available. No resources are reserved(not even for current unit production), production just pauses until new resources are available.

4.At builder unit location:builder unit spawns new unit(spawn/egg method):
A.Egg unit or proto-construct from which final unit is produced:
Typically spawning is fast and egg incubation time is long.
B.Spawn directly after incubation in unit/mobile structure.
e.g.Mobile vehicles spawning mines

5.From other unit:unit transforms into new unit type.(transmutation/unit-upgrade).
e.g. Mutalisk mutates into guardian, villager transforms to a swordsman.

6.At player unit panel, unit is bought and placed on the map where players has vision.

7.Unit is converted from enemy or neutral side:
Unit is either produced by enemy/neutral or pre-placed on the map. e.g. animal converted to player side.

8.Unit produced from spell/summon
RTS with RPG elements often use this method:
t.e.g. necromancer producing skeletons from fallen units.