Different schools have different policies on this. Stanford, for example, guarantees all legacies that their application will have 2 readers as insurance against a single reader in a bad mood. Imho, that’s not much of a boost and it makes it easier to tell a disappointed alum that junior got fair consideration.
Many schools will only give a legacy edge to ED applicants.
LACs, with smaller alumni #s and defined cultures, often value legacy. But remember too that this will be the deciding point between two equally qualified applicants. It’s not the finger on the scale that gets a mediocre student admitted over an excellent one.
One thing a lot of people forget is that the parent who went to xyz has probably stressed educational values and achievement that will make their kid a good applicant for xyz.
Major donors are a different class. Few alums can give at this level, and yes, if the kid will be able to do the work, they’ll probably be admitted over more stellar candidates. And fwiw, it is unlikely that such an application will suddenly appear at admissions. The donor will usually let the major gifts officer who handles the relationship know that their kid is applying and they may visit together and be shepherded to their AO. In some cases they may visit a few times over high school so that the student also understands what kind of application is required. (These kids should have nothing that makes the school say no.)
At most schools, if you are a legacy of any sort, admissions will let development know and they’ll provide admissions with guidance on level of giving and involvement so that admissions knows if this should be a factor. They often send the parents a letter thanking them for their vote of confidence.
At a very broad-brush level, I would say that plain old legacy can move a strong applicant to the accept pile but that you would need to be in a different and rarer group in the development office to get your kid (or grandkid) moved from meh to yes.