The Biblical prohibition in Squeezing fruits on Shabbos:
A. Edible Fruits:
Olives and grapes: One who squeezes olives in order to take out oil, or grapes in order to take out wine is liable due to that he has detached the juice from the fruit, and it was already explained in chapter 305 [Halacha 28] that anyone who detaches food or juice from the area where it was absorbed in, has [transgressed] an offshoot of [the] threshing [prohibition].
Other fruits: However other [edible] fruits are Biblically allowed to be squeezed to take out their juice.
The reason for this is: because fruit juices do not have the status of a liquid with exception to what comes out of olives and grapes alone. Rather they have a status of food on them and [squeezing them out] is like separating food from food which does not contain the prohibition of squeezing, as explained there [in chapter 305].
B. Inedible fruits:
First Opinion: The above is only referring to fruits which are fit to be eaten however fruits which are not fit to be eaten are forbidden to be squeezed according to all opinions even into food [and certainly not in order to drink their juice].
The reason for this is: because when one squeezes food which is not fit for eating he is not separating food from food but rather is [separating] food from waste which contains [the prohibition of] “detaching” as was explained in chapter 305 [Halacha 28]. It is also forbidden because of [the prohibition of] separating food from amongst waste, even if he wants to eat it right away, as [the Sages] only permitted to separate food from waste with ones hands in order to eat right away being that it is the form of eating to take the food with his hands from the waste and eat it, and [it is thus] not similar to [forbidden] work, however to squeeze and then eat is not a form of eating but is a form of work.
Second Opinion: There is an opinion which permits squeezing unripe grapes into food (and it goes without saying that they permit this to be done to other fruits which are not fit to be eaten, and even to squeeze them for their juice is permitted according to those who permit this with other fruits [which are edible]).
C. Grapes and olives which have already been crushed:
It is Rabinically forbidden to squeeze juice out of grapes and olives even if they have been crushed from before Shabbos and have juice flowing out from them.
Summary-The Biblical prohibition in Squeezing fruits on Shabbos:
Edible foods: The Biblical prohibition of squeezing juice from a solid is the Mifarek/detaching prohibition which is an offshoot of the threshing prohibition. It applies to squeezing juice from grapes and olives. It does not apply to squeezing the juice from any other edible foods.
Inedible foods: Regarding squeezing juice from inedible foods, it is disputed if it contains a Biblical prohibition or is even initially allowed. Practically one is to be stringent.
 Regarding inedible fruits refer to Halacha 2D regarding if squeezing them contains a Biblical prohibition.
 Meaning even according to the opinion which holds that fruits which are not commonly squeezed may be squeezed for their juice.
 However the Michaber brings two opinions on this matter regarding inedible grapes, the latter-stringent- opinion being like that of Rabeinu Tam. The Magen Avraham, Tosefes Shabbos, and Mishneh Berurah all rule like the latter opinion in Michaber, like the ruling here by Admur.
 Halacha 1