Where the summoned creatures come from?

In Elder Scrolls Oblivion and Morrowind you can summon a skeleton or a zombie to aid you in battle but where they come from?

I find logical other creatures such as imps, atronachs and dremoras are summonable, they live in another plane and this could be some kind of "pact" you entablish with them. I just don't know where those skeletons or zombies come from.

Solution 1:

I cannot say if this was ever explained in either Morrowind or Oblivion, but it is later explained in the Skyrim DLC Dawnguard. The Plane of Oblivion you are looking for is Soul Cairn.

It is a plane of Oblivion inhabited by souls and undead

Background

When a soul is trapped in a Black Soul Gem and its energy used to power an enchantment, the remnant—the soul itself—is sent to the Soul Cairn. It is a place devoid of life—the only beings that exist there are souls, undead and the rulers of the realm, the Ideal Masters

Solution 2:

Summoning is a skill in the conjuration school of magic. The skill summon, will summon creatures from other planes. So yes you are still summoning the zombies and skeletons from another plane.

Source: summoning creatures from other planes

Solution 3:

All summoned creatures come from one plane of Oblivion or another. This is why the symbol associated with the conjuration school is an Oblivion Gate.