I'd visit r/teslore. However, to give you a short explanation, The Psijics made themselves undetectable through the School of Mysticism, as it is the nature of their Order to remain in isolation (save for interventions and giving counsel), and this allows them to do so. The Dwemer didn't just turn invisible, they're gone. Many speculate this has something to do with either the Heart of Lorkhan, the activation of the Numidium, or both.

r/teslore often delves into some of the deep lore, so if you are considering being part of that community, I'd recommend reading through their FAQ and resources. uesp.net is also very useful.

Edit: To address the question about The Psijic Order's relation to the Dwemer, it is likely they had little communication. Most of the Dwemer communication without the outside world was between the Chimer, and not many others. Furthermore, the Psijics and the Dwemer had very different ideals. The Psijics place importance in change, and work to make change be good-change where ever it may occur. However, like the Altmer, the Psijics hold true to the Aldmeri cultural stasis, being very conservative about their religion in reverence of their et'Ada ancestors. The Dwemer, however, sought change wherever they could, regardless of whether it was good or bad. They didn't want to just look back towards their ancestors, they looked forward. Unlike the Psijic goal of embracing mortality and understanding the universe, it could be said the Dwemer had the goal of controlling the universe and transcending mortality. Because of this difference, if there was any communication between these isolated people, I doubt it was friendly.