Shalom, if a person forgot to do Kabbalat shabbes, and did arvit / mariv, did he fulfill shabbes? or should he do Kabbalat shabbes after arvit/mariv?
Thee are a few ways that one can be mekabel Shabbat. Either by saying Mizmor Shir during kabalat Shabbat, or if he didn’t say kabalat Shabbat, when the Chazzan says Borchu at arvit. Since you already davened arvit, you were already mekabel Shabbat with arvit.
O:CH 261-4, 263-10. See Toras Shabbos 263-25, brought in Shmiras Shabbos Khichoso 46 ftnt. 17, that if a person is davening B’yichidus it is considered being mekabel shabbos when he says אשר בדברו מעריב ערבים, since he is saying that it is not night, it is considered as if he is saying that it is the new day for him, which is Shabbos.