what is the difference between mindfulness meditations and focusing on other things? For example:
While I'm studying(or doing anything in my life), my brain wanders around(daydreams, stresses, remembers unpleasant memories and thinks negatively about the future) so I notice that and then bring it back to focus again in studying. Isn't that the same thing as mindfulness meditation? in both things, we focus on or attend to something and when the brain wanders around, we bring it back.
I read articles before 1 2  that say the difference is that in mindfulness, you let things(that you attended to) go after you finish without thinking about it again.
Does that mean that I can call studying a mindfulness meditation if I stopped thinking about it after finishing?
It is question I've been asking myself as well in my experience with mindfulness meditation.

Meditation is not manipulating experience, it is simple noticing it. Furthermore it focuses on sensory experience which seems to be inversely correlated with our narrative experience(Farb et al., 2007).

Being focused on studying involves planning and invariably it adds a narrative feature to our experience. Mindfulness increase grey matter in areas associated with executive control(Hölzel et al., 2011), hence we can hypothesize mindfulness is good for more effective studying. 

There might some type of false placebo effect in studies reporting increased focus due to meditation.

Farb, N. A. S., Segal, Z. V., Mayberg, H., Bean, J., McKeon, D., Fatima, Z., &amp; Anderson, A. K. (2007). Attending to the present: Mindfulness meditation reveals distinct neural modes of self-reference. Social Cognitive and Affective Neuroscience, 2(4), 313–322. https://doi.org/10.1093/scan/nsm030

Hölzel, B. K., Carmody, J., Vangel, M., Congleton, C., Yerramsetti, S. M., Gard, T., &amp; Lazar, S. W. (2011). Mindfulness practice leads to increases in regional brain gray matter density. Psychiatry Research, 191(1), 36–43. https://doi.org/10.1016/j.pscychresns.2010.08.006