For typical assembly processes, where you bond plastic to plastic or to other material components, you usually need a third material, the adhesive, to connect parts in a bonded joint.
Adhesive bonding is a very complex technology and not a simple solution. Plastic materials are subject to a combination of many different parameters and influences. However, there are some plastics that are easier to bond such as many amorphous plastics, for instance PC, PSU, PPSU, PES, PEI, PMMA, PS and some PI due to their partial dissolvability.
There are many specific assembly technologies available for industrial processes, depending on design conditions, number of parts, component size, application requirements and economy.