I like how nobody has mentioned riding style or terrain.
EDIT – ads678 has, but that wasn’t there when I was typing 😉
tyre pressues…the eternal mystery… for MTB between 10psi and 50psi depending on the following:
tyre casing width
tyre casing height
tread pattern
tread rubber compound
tubless or tubed
casing compliance
casing thickness
rider weight
rider skill
riding speed
rigid, hardtail or full sus
long or short travel
tarmac or dirt
which kind of dirt
whether there are rocks
big rocks or little rocks
gravel?
sand?
whether it is dry or wet
or icy
or snowy
or dusty
** other variables I’ve forgotten **
personal preference
mostly a good starting point for your average sized MTB tyre is 30F, 35R and then adjust either way to suit depending on above, just because some people swear by pressure X, doesn’t mean you will like it or get on with it.