Why does the longest chain rule make the most sense to deter double spending?

[Bitcoin]

Why does the longest chain rule make the most sense to deter double spending?

If there is double spending occuring, I don't understand why the longest chain rule would decrease the likelihood for only one transaction to be confirmed.

Can someone explain step by step why this makes sense logically? I've scoured through youtube videos but they do not explain the logic behind this. The videos only describes using the longest chain rule.

I also have one additional question. Are all the miners also verifying that the block that is solved is correct once it's solved?

Source: Why does the longest chain rule make the most sense to deter double spending?