How is it that when a court finds a government law or action unConstitutional that the decision only applies to those who brought the lawsuit?

I see this in federal as well as state cases.

An additional but related question: who has "standing" to, for example, halt construction on President Trump's addition to the White House? Who is being harmed by it? Yet state election fraud lawsuits in 2020/21 were dismissed out of hand because of their lack of standing.

Btw, I didn't mean my question to be rhetorical.

I understand that the Supremes limited nationwide injunctions. My question is about the theory behind a federal court declaring a law or regulation or government action unConstitutional, but only for the people who sued.

Obviously in other kinds of cases like I sue my neighbor for damaging my lawn, it is specific to the 2 of us. I'm referencing broad situations that apply to everyone, like banning a type of firearm or deporting a known illegal alien without an individual hearing.

And that I guess expands to what is the theory behind a federal circuit decision only applying to that area? How can it be decided law, presuming no appeal has been filed, but not nationwide? Or in the case of a state court regarding a state law, same thing that it only violates the state law or constitution related to the person who sued?

Is it merely precedent that some judge said that it works that way?