What happened when I join two different materials?
It's not about the number of electrons in a material, but about the number of excess or the lack of electrons in a material.

A negative charge is defined as an excess or electrons, and a positive charge as a dearth of electrons (or an excess of "holes" - the spaces where electrons should be).

Current flows when there are two materials with a difference in charges.  If you have a material with an excess of electrons and a material with a dearth of electrons (or an excess of holes) and they come into contact (or close enough for the insulating properties of air to be broken down) then the excess electrons in the first material will flow into the second material to fill the holes.

This is called "static electricity" because the initial charges are stationary (static) and contained within the materials until they come close enough for the electrons to flow.

In electronics we work with "active" electricity, which at the sub-atomic level is just the same as static electricity, except we use a power source to actively produce a difference in the number of electrons between two points in a circuit.

You can think of static electricity as a balloon full of air.  Burst the balloon and the air inside will rush out.  "Normal" electricity is like a fan - it's always trying to push the air through it.