I'm on PC. How do you check the floor level in the most recent minecraft?

Alt + f4

Use f3…

or fn + f3

On the Java version of Minecraft, you can check your position by pull up the debug info by hitting F3 (on some keyboards & laptops, you may need to hold down the FN button). This is NOT available in the Bedrock Engine versions.

The Y coordinate is showing your current "altitude" in the world from the bedrock base…

0 & lower is considered "The Void" (where the game has an insta-kill field located).
1 is the lowest you can safely reach in the world.
64 is considered "sea level"
128 is considered "cloud level" (the original maximum height in the game)
255 is the current maximum height in the world (game will not allow you to build any higher)
256 & higher is considered "outer space"