The types of Isekai change
There are mainly two ways for the character(s) to go to a different world.
One is "to die in their original world and to be born in a different world" (異世界転生, isekai tensei). In some stories, the character goes to the different world keeping their memory. But there are also stories that the character remembers their previous life after being grown up.
The other is "to move to a different world without dying" (異世界転移, isekai ten'i). The way of moving to a different world is different between stories. For example, the character may suddenly move to a different world, or be made to move to a different world by somebody like god.