Someone who finds fruit on the path under a tree that is planted along the path, if the fruit is of the sort that normally falls from the tree and when they fall they become disgusting, or even if not disgusting, but most of those who pass there are non-Jews, or they are fruit that it is normal for animals to eat and they [often] travel this path, then the owners already gave up on them and they are permitted.
However, if they are fruit that do not become disgusting when they fall, and most of those who pass there are Jews, they are prohibited because of theft.
If they belong to young orphans, they are prohibited in every way, since the giving up on [something] or their forgiving has no [halachic import; being minors, their intent doesn't have legal weight].
The law of the kingdom [ie the civil authorities of your country] is law.
Dina demalkhusa dina is a complicated topic, with long discussion by many rishonim as to its full scope. It is enigmatic that R’ Shelomo Ganzfried just quotes the idiom, rather than spelling out its details — it would could easily take an entire se’if (chapter)!
From its placement, we can deduce that the QSA considers dina demalkhusa to be primarily related to fiscal law and theft. But I can’t tell if he’s referring to stealing from the government, i.e. limiting the concept to taxation. Or if he is saying that property can be set by the customs defined by civil law and thus something someone else legally owns by local law may not be taken or used even if they wouldn’t have otherwise owned it according to halakhah.