Java. UUID, how does it works

The purpose of functions like randomUUID is to munge together various pieces of information which are likely to have some amount of randomness such that two UUIDs generated at different times or in different places would have an extremely small likelihood of matching unless all sources of potential randomness happened to yield identical results. No effort is made to keep track of which UUIDs have or have not been issued; instead, the goal is to have enough randomness that the probability of an unintentional match will be small relative to e.g. the probability of a computer being smashed to a million pieces by a meteor strike.

Note that the system may use "number of UUIDs issued" as part of its UUID calculation, but that would only be one of many factors that go into it. The purpose of such a counter isn't to allow one to "go back" to a previous UUID, but rather to ensure that if e.g. two UUIDs are requested nearly simultaneously without any source of randomness having become available between them, the two requests will yield different values.

UUIDs are unrelated to what file generates it. It doesn't matter which file generates them they more than likely will not be the same.

For question 2 the way UUIDs are generated doesn't really allow for them to be regenerated in any meaningful way. They are usually generated based on some info from your computer, the current time and other stuff. The Java algorithm uses a cryptographically secure random number generator and are known as type 4 UUIDs.