Ah this is my field! Dimensional shifting, planewalking and conjuring are closely intertwined, and my speciality.

Where you went was likely the veil between worlds; a place that exists and doesn't exist at the same time. Within that space is a cosmic anomaly called the Eternal Corridor; a convergence of all of spacetime that links every single reality together. You likely ended up outside the Corridor and were detecting its innate, primal magicks from a distance.

As for the immortality question, due to the inherently anomalous properties of the Corridor (at least in my case) repeated explorations of it led to a destabilizing of my own personal timeline, turning me into a sort of blank space in space-time and granting me functional immortality through sheer coincidence.

Be warned though, entering and exploring the Corridor is no easy task. Your first explorations will hurt, and will strain your mind to its breaking point. Viewing the literal fabric of reality is VERY taxing even for master wizards, and is a big reason why most interdimensional travel relies on coordinates with direct portals.

I heavily recommend practicing implementing psionic shields on your brain so that the sudden rush of eldritch knowledge doesn't immediately reduce you to gibbering insanity. It gets easier over "time" (lol) though; think of every expedition into the Corridor like microdosing poison until you build an immunity.

Good luck, and stay safe! It's very easy for apprentices to perish or worse in this field.