Joins path components using the platform path separator.
FileNameJoin[{"name1", "name2", …}]
FileNameJoin[{CloudObject[…], "name2", …}]
FileNameJoin[{LocalObject[…], "name2", …}]
FileNameJoin["name1", "name2", …]
FileNameJoin[{"a", "b", "c"}]
→ a/b/c