algebra precalculus - A Question About The First Step In Induction

I am currently learning mathematical induction from this site (https://www.mathsisfun.com/algebra/mathematical-induction.html). It has broken induction into 3 steps:

1. Show that it is true for n=1


2. Assume it is true for n=k


3. Show that it is true for n=k+1

I have 4 questions:

1. Why, of all numbers do we pick n=1? Can't we pick something like n=1, n=2, or the like?




2. Why do we need the 3rd step? I get a feeling it is to prove that it is true for all n=k, but if that is so, how does it do it? It does prove that it is true for all n=k+1, but that is based on the assumption that n=k; and therefore doesn't prove it. Because if a proof is based on an assumption, how does that prove anything?




3. Why do we need the first step when we show that it is true for all n=k+1?




4. In n=k+1, why do we add 1? Why can't we subtract 1, or add 2, etc? Why must it be n=k+1?

Is it possible to answer the question at the level of a Pre-Calc student, who hasn't learnt Calculus (obviously), set theory, and all those complicated stuff?

This question is different from "Dominoes and induction, or how does induction work?" because I have learnt neither limit notation nor L'Hopital's rule, and the other question contains them. This is important for a Precalc student who understands neither of them.