Cynimal | MCPK - How stepping works @Cynimal | Uploaded January 2021 | Updated October 2024, 2 hours ago.
This is bit of a random topic to cover, but it's often overlooked and leads nicely into explaining how blips, wall-blips, and jump-cancelling work, which are the topics of the next videos in this "series".

0:00 - Introduction
0:14 - Stepping up a slab
1:05 - Stepping up a block?
1:54 - Stepping up a repeater under a ceiling (1.8+)

The information in this video should be accurate up to version 1.13, as the collision code was reworked in 1.14 to "fix" blip-ups.
Pre-1.8 bug: bugs.mojang.com/browse/MC-3337

Note: a "bounding box" is a cuboid that represents the player's collision space (dimensions: 1.8x0.6x0.6). Every tick, it gets moved independently of the player's position to check for collisions with the environment. Blocks have a "collision box" that is composed of one or multiple bounding boxes.

I don't explain what "moving the bounding box down" does in detail, because that's the part which causes blips to exist (next video). For now, imagine it snaps to the nearest floor below it (within a reasonable range).

The code doesn't behave exactly as described in this video, but this is the simplest way visualize how it works, and it's basically equivalent. (in particular, the 1.8 method is done before the legacy method, but the order doesn't matter in the end,).