It is originally domain of the state w.r.t the mechanism put in place for declaration of Ramdhan and Eid. In the absence of such mechanism in place, Muslim communities living in a Non-Muslim majority country should make some kind of union to decide such matters. And in absence of such a union, one can opt following Saudi Arabia or can follow a scientific calendar in this regard.

Please refer to the video below from 18:09 to 21:33

https://youtu.be/BTvlixbSAT8?t=1089

Please also refer to the video below from 35:54 to 37:32

https://youtu.be/fG520W99Vc4?t=2154

Please also refer to the video below from 24:24 to 25:49

https://youtu.be/2OuABL8Yc64?t=1464