Identify this inlet?

[Oblivious]

Does anyone know what this white circular port at bottom of pond is for? There’s a sub tank for filtration (grate pictured) that outlets to waterfall but the port does not seem to be connected to either. I’ve reached my hand in and couldn’t feel a plug. There’s also this outlet above the water that is a mystery and possibly connected?

Thanks!

 

[GBBUDD]

Have you found the other end to these pipes?
PIC ONE I WOULD SAY WAS USED AS A BOTTOM DRAIN WHERE IT HAS A BASKET OF SORTS IN IT i would suspect it is to hold the strainer to the drain . " LITTLE MORE just a space around a cap look it it this way when you unscrew a cap to any bottle the more you turn it and open it up the more liquid that will pour out.

Pic two id say is the return to send water back into the pond .

Pic three i cant make out what your showing

Show a pic of the entire pond and your filter and pump