The electrons come from NADH or FADH2 (see red circles) ~ these are coming from Krebs cycle.
The electron carriers (NADH & FADH2) drop off their electrons at the ETC at special enzymes (called complex) that are imbedded inside the membrane of mitochondria (blue arrows).
The electrons will travel through 3 complexes (the picture is showing 3, sometimes its more) until finally they get accepted by 1/2 O2 to form water (yellow circles)
O2 is used at the very end of the ETC to make H2O. Also, there aren't any biproducts from the ETC per se because the purpose of the ETC is to reduce O2 to make H2O through alternating redox reactions in the complexes of the ETC. They don't generate any "compound". It's just a transfer of electrons and movement of protons. None of them go to the Krebs Cycle. The ETC uses the biproducts of the Krebs Cycle, if that's what you mean, like NADH and FADH2.