Basic summon command
A simple mob command looks like /summon minecraft:zombie ~ ~ ~. The entity ID and coordinates decide what appears and where it spawns.
Mob generator and spawn commands
Searches such as Minecraft mob generator, spawn mob command Minecraft, summon mob generator and Minecraft spawn command usually point to the same need: create a valid /summon command without writing all options by hand.
Items and non-mob entities
The summon command can also create some non-mob entities depending on version and edition. For dropped items, projectiles or special entities, check the generated command carefully because entity data is easy to break.