It pretty much says it in the video description there – Photoshop.
Take your 2D picture, use any selection tools you like and cut and paste onto separate layers the elements you wish to make ‘3D’. Then using clone/heal or copy paste or whatever, mend the background so it doesn’t have people shaped holes in. (Depending on motion, you might not need to do much – the original element will hide most of the hole)
Then bring the PSD into AE and move the element layers in 3D space and scale them so it looks normal. Add a Camera and move it and you have your (semi) 3D.
A good variation if you have strong perspective lines is to use the Vanishing point filter and save/import that as a VPE – can really give some depth.
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Only in after effects do children get to pick and whip their parents.
https://hennell-online.co.uk