No. An adult *hand* can fit into *some* women’s vaginas, although it’s a pretty highly skilled sex act and the majority won’t want to or be able to do it. But a full arm? No.
Technically it can. But not any woman. It requires practice. You can't just do it to anyone. It's like running a marathon. It can be done people so it all the time but not everyone can do it.
during birth, gynecologists have reached in there to turn the baby if it's breached. it can get about half way up the arm, maybe the elbow. back in the day, that probably happened, doubt if it happens now.