I struggle to determine the exact differences e.g. between CYPD2121 and CYPD2122, is there any better explanation available than table 27 in the datasheet?
I intend to create two devices
a) portable equipment with battery that is charged using USB-PD but may source back to attached equipment
b) ethernet to USB adapter that is operated on the portable equipment but may source it if PoE is available
--> can I use CYPD2122-20FNXIT or -24LQXI depending on which package I prefer for both designs?
Yes you can use depending on which package you prefer to the designs. Since CYPD2122 and CYPD2121 is target for Dual Role Power.
BTW, I would like to recommend you use CYPD3171 for the Dual Role Power Design.