There is no opening that requires little to no study. Only openings that have fewer variations to deal with.
For example, 1.b4 requires knowing far fewer variations than 1.e4, but you still have to know 1...a5, 1...c6, 1...d5 with 2...e6, 1...d5 and the Bishop comes out, 1...e5 with 2...f6, 2...d6, and 2...Bxb4, and then Kings Indian and Dutch setups.
Granted, far less than the Dragon, Najdorf, Classical, Scheveningen, Taimanov, Sveshnikov, Kalashnikov, Four Knights Sicilian, Pin Variation, Nimzowitsch, O'Kelly, Ruy Lopez, Petroff, Philidor, Latvian Gambit, Elephant Gambit, French, Caro-Kann, Pirc, Modern, Scandinavian, Alekhine, Owen's, St George, Borg, etc.
But you still need to know theory, and really ideas from other openings. The Sokolsky, 1.b4, only has 1 real independent line, the Exchange Variation (1.b4 e5 2.Bb2 Bxb4 3.Bxe5). The rest have massive similarities to other openings. You get a lot of English Opening type setups, a lot of Slav and Queens Gambit type setups, a lot of Kings Indian type setups, and even the reversed French in the 2...f6 line (1.b4 e5 2.Bb2 f6 3.b5 d5 4.e3 Be6 5.d4 e4 with a reversed Advance French with ...f6 committed, weakening the diagonal after White is able to get in c4, and he will likely have to move it again to f5, gaining White a second tempo yo go along with the one he gained for going first.)
This is all far less work than that of 1.e4, but you still are not getting away with "little to no study".
What is the EASIEST opening that requires almost no study?
I will share my idea: simply fiancetto the bishop.
https://youtu.be/W0gIU1JR4Ug?si=lH6-pLiSiamBBOx4
This is a great idea for players under 1400 on here, although at higher levels you obviously need to know more than that.
What do you recommend for beginners and casual players?
What do you recommend for higher levels, like 1800 or 2000 or 2200?