Firstly, I complete my full workout, and after that, I give my proper time to cardio and abs training. Personally, I do crunches, planks, climbers, and cardio as well. But most important for abs is your diet so that you get a lean body or less fat.
I will suggest you to do one muscle group a day(5days) but with that regularly do proper cardio and can do abs 2-3 days.
1st day-Back
2nd day-Chest
3rd day-Legs
4th day-Shoulder
5th day-Arms
No, you don't, but there is no benefit to it at all; rather, it will only cause you injury because your upper body will get stronger and your legs will remain the same, so there is no point in it. Your body will also look bad because your upper body looks masculine and your legs look thin. Train...
Dude you can change your workout routine if you get bored. Like i mostly prefer one muscle group a day so that muscle will get enough time to recover. Previously i also get bored from this routine so i get shifted to push pull leg workout and you know it also helped me in gaining strength so...
Machines are good, but if you are intermediate or above, you have to do exercises with dumbbells or rods as well. It will provide you with an extra boost in making muscles, and it will be very effective. Everything has its advantages and disadvantages, so that's why i use both.