I think he ment jump height as in a value that determines how many units you jump up if you press the jump button not the y position.
So you could do a hack like superjump.
Same thing for velocity, standing still it's 0 but you do not know the max speed you can go in float.
I think you could find things like these out with cheatengines "find out what accesses/writes to this adress"
Or just pure luck of editing floats in the mem viewer.
But im not a really good mem editor, i would call myself above average.
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added 3 minutes later
Oh for velocity try this, might work, might not.
1. Autopause game to yes
2. Stand still
3. Search 0 Float
4. Run a bit and while running press gg icon and go to unknown-> Value increased
5. Stand still again and repeat until you get only a handful of values. Freeze them to a high number one by one to see if it changes your speed. If a value doesnt affect your speed unfreeze it, revert and delete it so you dont break any ingame values.