What do Birth, Age, and Death mean in the first noble truth?
Here, "X is subject to birth, aging... illness... death... sorrow... defilement" - means that X has beginning and end, that X changes with time and slowly falls apart, that X depends on external conditions to be just right for its existence, - and that therefore X is not 100% reliable, so you can't use X as foundation for building 100% stable peace and happiness. That X is never really what it looks like, so if you build your happiness on top of X, it will betray and disappoint you. Even if association with X may seem like bringing happiness on the short term, on the long term it will definitely bring eventual suffering. That's what this means.

Gold and silver are there as examples of worldly pursuits and worldly possessions. No matter how much you have, you can't buy happiness, can't buy health, and can't buy eternal life. At their best they can only bring intermittent short-term happiness mixed with troubles and pain, ending in eventual destruction.

So Birth here is coming together of any form (samskara) - and Aging-and-Death is its falling apart. Bhava is the continuity and cohesiveness we imagine in between the two.

Births and Deaths are not dukkha if there is no attachment: craving, grasping, denying - either "for" or "against". Attachment, in turn, comes from getting stuck on a certain mental context. If you stop getting stuck you can learn to be free. This is what they call Unbinding.

Not being stuck means things are just such, regardless of births and deaths. Being wrong against some definition of right is the nature of dukkha. Being just such is the nature of peace.