The term in copyright law for such a "platform" is a "collective work" or "compilations ". US law, 17 USC 101, says:
A “collective work” is a work, such as a periodical issue, anthology, or encyclopedia, in which a number of contributions, constituting separate and independent works in themselves, are assembled into a collective whole.
The laws of other countries are similar in this respect.
Each individual item in a collective work has a separate copyright, held by the author, author's employer, or whoever the copyright may have been transferred to, as the case may be. There is a copyright in the collective work itself. This protects the selection and arrangement of the individual items, as well as editorial matter such as an introduction, index, and notes on individual items furnished as part of the collection.
Even when individual items are in the public domain, the collection may have a copyright. For example, if someone prepares a CD of Best Bach Fugues the individual compositions are of course in the public domain, and the recordings might well be, depending on the circumstances. But the choice of which fugues are "Best", which recordings of those fugues to select, and what order to p-resent them in are all the creative work of to compiler, and those are protected by copyright. If some other person published a CD of Great Bach Fugues copying the selection and arrangement of the individual recordings, that would be an infringement (unless permission had been obtained). This is true only when there is creative effort in the selection and arrangement. A collection of all known Bach fugues, arranged chronologically, would be a "natural or obvious" arrangement, and not protected by copyright, just as would a complete collection of the works of Dickens, say, arranged in alphabetical order. See Feist vs Rural for more on the degree of originality needed in a collection.
Of course, sometimes the publisher of a collection obtains the copyrights of individual items. It was once common for magazines to purchase the copyright of articles that they published, but now it is more common that they merely obtain a limited license for their use. But the copyrights are still separate, even if the holder is the same.
To quote the US law on the topic, 17 USC 103 provides that:
(a) The subject matter of copyright as specified by section 102 includes compilations and derivative works, but protection for a work employing preexisting material in which copyright subsists does not extend to any part of the work in which such material has been used unlawfully.
(b) The copyright in a compilation or derivative work extends only to the material contributed by the author of such work, as distinguished from the preexisting material employed in the work, and does not imply any exclusive right in the preexisting material. The copyright in such work is independent of, and does not affect or enlarge the scope, duration, ownership, or subsistence of, any copyright protection in the preexisting material.
M<y understanding is that on this point the laws of most other countries are similar.
