In Legends, the initial construction of the Death Star battle station was done by the
Separatists before it was later taken and completed by the Galactic Empire.
However, in Canon, the initial construction of the Death Star was actually done by the
Republic, after its plans ended up in the hands of Republic scientists and engineers.
Although in reality the plans were secretly handed to Palpatine by Dooku, within the hallways
of the Republic weapons committees it was believed the plans were stolen from the Separatists
after they retreated from Geonosis.
The acquisition of these plans both terrified and fascinated the engineers and high ranking
military officials of the Republic who were privately briefed about them.
It terrified them as they feared the Separatists building the very weapon and using it to win
the war.
It fascinated them as they believed they were capable of building the weapon first and using
it to win the war for themselves.
It essentially sparked a secret arms race mindset inside the Republic, which is possibly
what Palpatine had hoped for after he leaked the plans.
Multiple meetings were held in the Republic Center for Military Operations to discuss
the logistics of constructing such a weapon and later the progress of it once it was approved
for construction.
Only the inner core of the Republic’s greatest minds and most politically influential were
allowed in these meetings, including Mas Amedda, Doctor Gubacher, Orson Krennic, and of course
Palpatine himself.
Everyone who attended the meetings had to sign an oath of secrecy of everything discussed.
The Jedi were of course not invited to these meetings, and it was actually made clear to
keep it a secret from them too.
Because the project was to remain an absolute secret from the public and to even the majority
of the military, private channels were created to secure necessary funding for the weapon.
Multiple, separate research entities and engineering projects were created across the galaxy to
further disguise their true intentions of eventually all being combined for the creation
of the death star battlestation.
So for example, a project was created for mass gravity stabilization, in which the engineers
were told that this project’s work was going to be used for massive medical space stations.
Other research centers were created for energy production, which Galen Erso eventually became
apart of, with him only being told that the project’s purpose was to provide sustainable
energy to wartorn and impoverished worlds.
In reality all of these separate, small projects were primarily developed for the sole purpose
of eventually bringing them all together to form the Death Star battlestation.
The construction of the main platforms and outer shell of the death star began over Geonosis
following the Republic’s second conquest of the planet.
Geonosis was chosen for a variety of reasons, mainly stemming from the deal the Republic
made with Poggle the Lesser after he was captured earlier.
It was estimated that in order to construct the exterior portions of the battlestation
it would require the labor of millions of workers.
In hopes of keeping the costs down and keeping the project a secret, the Republic negotiated
with Poggle the Lesser to have the tens of millions of Geonosians that obeyed his rule
do the strenuous physical labor in exchange for his freedom.
He accepted the deal and thus construction started over Geonosis.
Other benefits of having the construction take place over the red planet was the abundance
of raw material among the rings in orbit that was easily accessible to be used for the battlestation’s
construction.
Plus the Geonosians themselves already had experience in engineering from the droid factories
that they built, so little to no additional training was necessary among the workforce.
The initial stages of the construction were for the most part successful, with only minor
problems arising from small riots and conflicts from the Geonosian workers.
However, Poggle reassured his superiors that it was natural for the workers to act out
every once in a while and that it was within their culture to do so.
In reality, Poggle purposely encouraged these riots so he could one day use them as a distraction
for his escape into Separatist controlled space.
Despite his escape, the construction still continued.
Nearing the third year of the clone wars, a war that continued to devastate countless
worlds and bankrupt the Republic, higher ups began to get desperate in finding ways to
end the galactic wide conflict.
On top of fears of the Separatists creating their own planet destroying weapon, most people
believed the war would go on for decades.
As a result, serious talks were taken among high Republic officials about using the Death
Star immediately after it was completed to destroy the Separatist capital planet, in
hopes of it being enough to end the war.
Though of course the war never went on for decades, lasting only about 3 years.
After the creation of the Empire, no evidence of a Separatist Death Star construction site
was found.
Nonetheless, the construction of the original battlestation continued under Imperial supervision.