But when Order 66 was executed, it was Ahsoka who helped to free Rex from the programming of his inhibitor chip. At the dawn of the Empire, Rex has escaped with his life but is considered killed-in-action in the eyes of the new regime.
At the time order 66 was issued Rex was with Anakin's former Jedi apprentice Ahsoka Tano on Mandalore. They were there with a battalion of clone troopers to overthrow the rule of the Sith Lord, Darth Maul , who had taken over much of the planet.
Rex took Fives' warnings about the inhibitor chips seriously and made an official report on the matter. This knowledge allowed Rex to resist Order 66, and Ahsoka Tano was able to stun him and remove his chip.
As night fell, Rex met with Skywalker, Windu, and Boll at Urus's palace to discuss the problem of the uncontrollable Zillo Beast with Palpatine. While the Dugs wanted to kill off the Zillo Beast, the Jedi wanted to preserve the creature, as it was one of the last of its near-extinct species.
The “Ahsoka” series is expected to be set around the same time as “The Mandalorian” – almost eight years after the events of “Return of the Jedi,” by which time the Rex character will be a chronological age of 44 and a biological age/appearance of a man well into his 80s (Morrison is 62).
Rex tells Jesse he has the situation under control, but Jesse says he must follow orders, and if Rex doesn't, he will. Rex reasons that Order 66 was to execute the Jedi, but Ahsoka is no longer a Jedi.
Some clones disobeyed the infamous order than decimated the Jedi order, and refused to execute it. SOME of these Clones include; Captain Rex: Captain Rex, as most know removed his chip that would later be the way that Palpatine would execute order 66. Wolffe: As with Captain Rex Wolffe removed his bio chip.
A few clones were immune to the order, but they were quite rare. Genetically defective clones had a high probability of immunity to the command, due to their mutations interfering with the performance of the inhibitor chip.
For most of the Clone Wars, the 501st Legion was led by Jedi General Anakin Skywalker, Jedi Commander Ahsoka Tano, and Clone Captain Rex, who served as their first in command.
These clones help the crew of the Ghost in their fight against the Empire, and eventually join the Rebel Alliance. Rebels established that in the years after the Clone Wars, the Empire gradually replaced the aging clones with the less effective but more numerous stormtroopers.
Darth Vader's true identity as Anakin Skywalker was a highly guarded secret, known only to a select few individuals including Palpatine, Obi-Wan Kenobi, Luke, Han, Leia, R2-D2, C-3PO, Master Yoda, Bail Organa, Ahsoka Tano, Darth Maul, Grand Moff Tarkin, Grand Admiral Thrawn, Jocasta Nu, Ferren Barr, Verla, Reva, ...
But before the battle for Coruscant, they had to split up and Rex went with Ahsoka to Mandalore, and Anakin went to Coruscant. After that, Anakin and Rex never saw each other again.
R2-D2 was present when Padmé gave birth to Leia and Luke, so he knew Luke was Anakin's son – but what he didn't know was that Anakin became Darth Vader.
Rex is actually 31 years old, though his biological state is likely closer to at least sixty years, due to the accelerated aging that was genetically installed in all Clone Troopers. Because of their accelerating aging, Clones age twice as faster than regular humans.
This indicates that Rex hasn't interacted with Obi-Wan since the Clone Wars, making an appearance by Rex in Obi-Wan Kenobi impossible without contradicting Rebels.
Anyway, in answer to your second question, Rex is not a Mandalorian. His armor bears a resemblance to Mandalorian armor, however, because the clone template Jango Fett wore Mandalorian armor, and the armor of clone troopers was based upon Fett's.
Vader did not hunt for Yoda in any cinematic depiction. He and Sidious could not find Yoda as he had exiled himself on the planet Dagobah which had strong dark side energy that masked Yoda's presence in the Force.
Within the story, a pirate named Gruk has taken a crewmember of the Star Wars pirate Sidon Ithano (aka the Crimson Corsair) - Quiggold. The hostage quickly informs his captor of why his plan is doomed to fail - Kix (now referred to as "the Last Clone") has infiltrated their ship.
