Telepathy, teleportation, flight, energy projection, all part of the standard 'Eternal' base package, so he probably got those from the Celestial enhancements. I don't know if it's ever been clearly stated so, but it would make sense. He started out with a super-tough/super-stretchy/control my own molecules sort of power, and then discovered Celestial tech, and now has a bunch of powers that look kinda like Eternal powers (another Celestial creation).
I haven't seem him do Sersi-style transmutation yet, but most Eternals can't even keep up with that, she's a bit of a prodigy.
Apoclaypse is an External, not an Eternal.
His telepathy is limited to just that, telepathic conversation. He doesn't have the skill of any proper telepath and while he is skilled at astral combat, he requires help (either mechanical as seen in the Blue Area of the Moon Saga or a skilled telepath's help like Cable as seen in Onslaught) or he can't go to the Astral Plane.
Which is kinda what I said. He's not *actually* an Eternal, but his Celestial-enhanced powerset includes a lot of what the Celestial-enhanced Eternals can do. Which is convenient.
And their telepathy, as you say, is mostly used for telepathic conversation, it's not X-Men level telepathy, which is a dozen different powers rolled into one. The average Eternal isn't Emma or Jean or Xavier, able to do stuff like absorb or teach skills or modify memories or swap bodies or mind control people or create psychic illusions or mind-blast people. They just chat without using their mouths, for the most part.
I just remembered that Celestial from Black Vortex with a very suspicious design.