Where did the apes find another Icarus spaceship from to return to earth ? & How did they have the knowledge to pilot it.
In the movie "Escape from the Planet of the Apes" (1971), three apes — Cornelius (Roddy McDowall), Zira (Kim Hunter), and Dr. Milo (Sal Mineo) — escape the destruction of Earth by salvaging and repairing Taylor's spaceship (i.e. the spaceship Icarus, as seen in the original 1968 film).
As to how they knew how to pilot it, the film does not say (such events occur off-screen). But, as these were highly intelligent apes, they presumably "worked it out". They pilot it through the shock wave of Earth's destruction, and journey through a time warp.
It is possible that they found a third ship in a hangar intact and ready to go.
Charlton hestons (Taylor) Icarus sunk at the cross of father's canyon lake. James Francesco,s ship did not turn up to the battle beneath the planet of the apes. The ship that returned to earth was way before the mentioned movie above.