What are the ways to implement free cross-posting from wp to vk?

Question

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Good day.
Dear toasters, you need to implement cross-spotting of photos from Instagram to the WP site and from it to the VK group.
Preferably without resorting to paid services. Ideally, make your own service or application that will do this.
Familiar with JS, HTML CSS.
I don’t understand this topic (Crossposting) well, I don’t know which side to take.
Tell me where to start, what to study, if there are examples of ready-made solutions - it will be just wonderful.
Thanks in advance.
Happy Victory Day!

Answer 1

with VK API
and WordPress