Embodiments of 3D memory devices and methods for forming the same are disclosed. In an example, a method for forming a 3D memory device is disclosed. A first channel structure and a second channel structure each extending vertically through a memory stack including interleaved conductor layers and dielectric layers are formed above a first substrate. A semiconductor connection is formed above the memory stack and in contact with one end of the first channel structure and one end of the second channel structure. The first substrate and a second substrate are joined. The first substrate is removed to expose another end of the first channel structure and another end of the second channel structure. A first semiconductor plug is formed at the another end of the first channel structure, and a second semiconductor plug is formed at the another end of the second channel structure.