What are the basis of a good perish trap team? and what pokemon fit in the archetype? (not including goth and lapras)
Perish Trap is a really difficult strategy to pull off in this generation due to the lack of Mega Gengar, the dominance of Ghost-types, and the overall power level making things difficult to control. In all honesty, if you want to have a team with some viability outside of Perish Song I just recommend Lapras + Gothitelle + Incineroar and balancing it from there.
That being said, if you want to make a fully fleshed out Perish team, I would consider ways to control the opponent's damage output (Veil does this, but also things like Fake Out, Redirection, Intimidate) and ways to keep your own Pokemon healthy (Heal Pulse/Floral Healing, recovery items, etc). you're also gonna have to keep Gothitelle on the field often with things like Eject Button and pivot moves (maybe try Gothorita if you really want to tap into the trap aspect).
Some sort of offense is still appreciated, because Dragapult/Spectrier will likely run circles around you without it.