The melakha of Kotzer (reaping) involves detaching something from its source of growth. Since this is generally done to meet long-term needs, it is considered melekhet avoda and may not be done on Yom Tov as on Shabbat. One may not even pick some fruit to serve at a Yom Tov meal.
The melakha of Dash (threshing) involves separating grain kernels from their stalks, or any similar activity such as shelling peas or beans. This melakha is generally accomplished using machinery in a field or factory. Since it is generally done in bulk, it is considered melekhet avoda and may not be done on Yom Tov, even for Yom Tov needs. However, if the shelling is done by hand, it is impossible to perform this melakha in bulk, so it is not deemed melekhet avoda and may be done on Yom Tov. Thus, wheat kernels may be separated by hand, and peas and beans may be shelled by hand. No shinui is necessary; it may be done whichever way is most convenient.
There is a tolada of Dash called Mefarek, which involves extracting one thing from within another thing. This includes squeezing grapes to make wine and squeezing olives to make oil – that is, extracting a liquid from solid food. Since this melakha is usually done for the long term, it is considered melekhet avoda and may not be done on Yom Tov as on Shabbat; all the details are the same for Yom Tov as Shabbat. The main rules in brief are as follows. Fruit may not be juiced into liquid form, but a lemon may be squeezed into solid food like salad or onto fish, because the liquid is transferred directly from one solid food (the lemon) into another, and never has an independent status (Peninei Halakha: Shabbat 12:8). Milking a cow constitutes a violation of this melakha (ibid. 20:4), and the details of its laws on Yom Tov are the same as those of Shabbat (SA 505:1; see Peninei Halakha: Shabbat 11:17 and 28:7).
. Some permit juicing fruit on Yom Tov with a shinui based on two principles: 1) They maintain that the laws of Seḥita are like the laws of Toḥen. Just as milling spices for home use is permitted (section 2 below), so is squeezing juice. 2) According to about half of the poskim, the only squeezing that the Torah prohibits is that of olives or grapes. The prohibition on squeezing any other fruit is rabbinic (Peninei Halakha: Shabbat ch. 12 n.13). Since a number of Rishonim (and SA) maintain that on Yom Tov the entire prohibition on Mefarek is rabbinic (see 3:2 above), we should not prohibit squeezing fruit other than olives and grapes, as that would be a protective fence around a protective fence (“gezeira li-gezeira”). Indeed, this is the bottom-line ruling of She’arim Metzuyanim Be-halakha 98:7 and Shemesh U-magen 2, OḤ 30; Ḥelkat Yaakov 2:85 is permissive in pressing circumstances. In contrast, most poskim prohibit all juicing: SA 495:2; MA 495:3 following Yam Shel Shlomo; Ḥayei Adam 81:7; MB 495:12; SSK ch. 5 n. 1 in the name of R. Shlomo Zalman Auerbach; Or Le-Tziyon 3:19 n. 5; Halikhot Olam vol. 4 p. 100. This is because fruit is generally juiced in sufficient quantities to last a long time, and thus it is always prohibited on Yom Tov. In contrast, grinding and separating are regularly done for at-home use, during or slightly before eating, and therefore they are sometimes permitted. See Harḥavot to Shabbat 12:8:5 on the topic of juicing a lemon for lemonade. Some poskim are lenient even on Shabbat, while most are stringent. Accordingly, I follow the stringent position. There would seem to be room to be more lenient on Yom Tov, but in fact all poskim who prohibit juicing fruit on Yom Tov prohibit squeezing a lemon as well.