Sideloaded books and Kobo app

[Believe82]

Hello I have been for the past year using kobo app very nicely on my sony phone and have a mixture of bought books from kobo and sideloaded some books from my pc. Before I had kobo I used to have a nice folder with all my sideloaded books. Since I got kobo this folder has been clean empty.
My questions comes now, I got a new phone and I would like all my sideloaded books transfered there. I know that kobo won't sync them as they are sideloaded, but shouldn't they be stored on some folder on my phone?
I thought that if I could find them, then I could transfer them via dropbox or bluetooth.

Thanks