How much of a meme is "deadlifts and squats train abs anyway so don't bother isolating them"?
Because my friend and I can get to similar levels of body fat, my arms and legs have more visible separations than his yet his abs will be far more visible? Should I start training them on their own?
Pretty much a meme. Train them.
As for the you and your friend, he might store more fat on his arms why you store more on your belly, and the result is what you described. But he might have them developed better, so they pop more. Hard to say.
You should cut out squats and do stomach vaccums along with heavy OHP and DL for abs
Shit like sit ups and leg raises aren't very good and bad for posture
Pullovers using plates are also pretty good for abs
I'd say a good deal if you care about overall strength or aesthetics. There's a reason why bodybuilders isolate abs, and why strongmen isolate biceps for that matter. I think the idea that only a powerlifting or SS-type routine is legitimate is an overreaction to the uninformed novice lifters that base their routine on isolation exercises and don't use compound movements. A lot of these guys probably started out in that boat.
Heavy squatting and deadlifting does work your abs but not nearly to the same extend as isolated ab training does. But honestly it's a fairly straight forward issue to handle - Are you happy with the strength and size of your abs? If not, work them harder.
On a semi-related note, please realize that bodyfat percentage is an average of how much fat you carry on your whole body. Two people with similar bf% can look very different. One may have lean legs but a bit of a gut, while the other may have chiseled abs but a big fat ass.
Everyone should do planks daily, regardless of sport or the rest of their routine. Absolutely no reason not to.
Squats and stuff work your core, but not that much, not for very long, and rarely are most of your core muscles straining during those lifts. It's just usually not enough volume to have a truly strong core without doing planks and such.
You probably do pullovers wrong. You are not supposed to move your elbows at all during a pullover, so consequently you don't work your triceps. It's mainly a lats exercise, but you also kind of work your chest, which makes it a rather unusual exercise.