From what I have heard, it sounds like the next-gen Xbox players will not be able to play online with PS4 players. Is this correct? I really don't understand why, if it's true. Apex Legends and Call of Duty are crossplay and PS4, PS5, XB1, and Series X can all play with each other!
no every system will be able to play each other
Yes, someone on Series X can play with someone on PS4, just like someone on PS5 can play with someone on Xbox One X. It doesn't matter.