I still not understand at this day about Tidus being a projection of dead souls or whatever.
Is him an actual person or just a "spirit"?
It's all a bit metaphysical but... Right, so the FFX world is filled with these things called Pyreflies, which are like will-o-wisps or like pieces of soul energy. They can carry the will and memories of people, and when collected together can form physical manifestations. Fiends are formed when Pyreflies carrying the grudges, regrets, or other negative emotions of the deceased collect together, similar to vengeful ghosts in old Japanese superstition. Unsent are formed when someone has a strong will and refuses to die that their will is able to pull surrounding Pyreflies together to form a new body out of their self-image, essentially becoming a physical ghost.
Now there's some secret ritual that allows you to take someone's soul and place it inside a statue, which forms a Fayth (super literally a "praying child" in the Japanese version). The experience of being a Fayth is likened to being asleep and dreaming. Aeons are formed from the imaginations of Fayth thinking up their own image of a powerful guardian to aid summoners, their constant "dreaming" being the mental/emotional energy that glues the Pyreflies together into the summoned Aeons. Now, Sin and Tidus's Zanarkand (which is actually just floating out in the middle of the ocean somewhere) are effectively the same as Aeons, but instead of a single Fayth they're formed from an army of Fayth made up of the former population of the real Zanarkand, lead by Zanarkand's final ruler Yu Yevon. Tidus is one those, he's not made of regular physical matter, but of Pyreflies held together by the collective dream of Zanarkand and its people.
The Fayth, as the Bahamut kid Fayth explains, are tired of dreaming of Zanarkand after 1000 years, but they can't stop unless Yu Yeven is gone. Yu Yevon was so single-mindedly obessed with the idea of making Zanarkand live on forever that his obsessions have remained as the core of Sin that keeps everything still going. When the heroes obliterate Yu Yevon at the very end, this allows all the Fayth to finally stop dreaming and attain true rest. This means the Aeons, Sin, and Zanarkand all disappear, and also means Tidus disappears too, as the Pyreflies making them up can no longer be held together into physical form.
Now even though the popular of Dream Zanarkand are basically like Aeons, made from the Fayths' imagination, it's implied they have enough autonomous will that they have their own souls after death, which is why Tidus was able to see his mother in Farplane, and why there's that symbolic visual at the end showing Tidus and Jecht reuniting in the afterlife. In X-2 we have Shuyin, who looks exactly like Tidus and was a famous Blitzball player in the real Zanarkand 1000 years ago. The implication is that the Fayth based Tidus off Shuyin, but Tidus isn't a 1:1 copy of Shuyin, it's more like his general appearance and some basic manurisms were taken from Shuyin because those were things the Fayth knew of Shuyin as a celebrity.
At the end of X-2, Tidus comes back because the Fayth, as thanks to Yuna, are willing to dream for one lifetime longer to reform Tidus's body so he and Yuna can have a life together.
There's also some more stuff from the X-2.5 novel, where there's another type of summoning made from the summoner's imagination, and Tidus is revealed to actually be one of these formed from Yuna's memory of him and he could think himself out of existence if he ever become self-aware of this and has an existential crisis, kind of like what happened to Alucard in Hellsing. But that's all a bit silly and trying to set up stuff for a X-3 that will probably not happen because it's a bad idea, so take that as you will.
