Plinko is not rigged. The game works on basic physics and probability, both in physical arcades and online casinos.
I have played many Plinko games and studied how they work.
Physical Plinko uses a board with pegs and a plastic chip. The chip bounces off these pegs as it falls. Each bounce changes the chip's direction based on where it hits the peg. This makes the final landing spot hard to predict.
Online Plinko uses random number generators (RNG) to copy real physics. These RNG systems get tested by gaming authorities to ensure fair play. Licensed online casinos like Rainbet must prove their Plinko games are random and not manipulated.