So... I just read the descriptions of these "transitive" planes, and I understood what they were - planes that connect other planes, but how am I supposed to imagine them? PHB gives a visual description, but I couldn't quite catch it. For example, the Astral Plane is a "silvery sea, the same above and below, with swirling whisps of white and gray streaking among motes of light resembling distant stars." Alright, a sea... but where does it connect with the 16 Outer Planes? Are there doors, like in the Outlands? Or are there just holes tearing through space-time, leading to Bytopia or the Abyss? What about the Ethereal Plane? A misty ocean, with shores (Border Ethereal) that are practically like another dimension overlapping the Material Plane (couldn't imagine it any other way, given how with true seeing and other spells you can see creatures in the Ethereal Plane). Same question: where does this Plane connect with the other Inner Planes? Are there doors? Random areas that teleport you there? Gates? Where are these gates/portals? Please help me.

Take a look at the DMG when it comes to figuring out planar placement. But you might find this passage helpful.

Since the primary way of traveling from plane to plane, even using the Transitive Planes, is through magical portals that link planes together, the exact relationship of different planes to one another is largely a theoretical concern. No being in the multiverse can look down and see the planes in their arrangement the same way as we look at a diagram in a book.

Edit: Beans! That didn't answer your question. The astral plane has color pools, basically portals to the outer planes, the material plane, and the ethereal plane. Deep Ethereal has "Ethereal Curtains" that lead to the inner planes. In both cases the placement is pretty random.