It depends if you’re talking about devoting an entire day to “core workout” or if you just want to find ways to work some “core” into the rest of your routine, I think.
I don’t really do much direct “core” work, but I do a yoga class or two every week, and within my workouts I’ll occasionally include loaded carries (waiter walks are an absolute killer; suitcase walks are also fantastic; both of these can be done with a dumbbell or kettlebell).
But if you’re looking for more direct core work, there are some good articles on this site:
https://www.t-nation.com/training/core-training-reality-check