How to find the End portal (stronghold) in Minecraft
The End portal is hidden in a stronghold deep underground, and eyes of ender lead you to it.

The End Portal is the starry gateway to the End, opened in a stronghold.
It gives off light level 15. It cannot be broken in Survival.
It appears when all twelve End Portal Frames of a stronghold portal room hold an Eye of Ender. Stepping in sends you to the End island where the Ender Dragon waits. The portal block cannot be collected, and a new exit portal appears after the dragon dies.
This item has no crafting recipe.
The End portal is hidden in a stronghold deep underground, and eyes of ender lead you to it.
An End portal has 12 frames, so you may need up to 12 eyes, plus spares for the ones that break when thrown.
It takes you to the End.
Not in Survival: its hardness is -1, so no tool can mine it. Its blast resistance (3,600,000) also stops explosions.
Yes, light level 15.