To complete this infinite height glitch, you must,

1.Start flying with an elytra on a flat world.

2.Start holding the A button with the Right Nintendo Switch controller.

3.Sustain holding the A button and you will go straight up to incredible heights without needing elytra fireworks.

Also, you do not need to look directly up. My brother discovered this, and he got to Y height 25,000 without rockets. This glitch works with any Nintendo Switch controller including the Pro Controller and the Wired pro Controller.