This has been documented quite extensively & you can look up a full picture quite easily, but in a nutshell - the Metis descended from a relatively small number of Indigenous women who were partnered with Euro men early-mid 1700s. You cannot assume every or most forever then on somehow 'married' another white man. In fact, there may not have been another 'white man/woman' ever in  many families. 

These original Metis moved to  western areas - Red River/now Winnipeg and throughout the prairies, some groups to Quebec. These women's descendants were very well-documented and along with identifying as Metis, the Metis Nation Assns require descendants to provide records that show those connections which these societies will research for authenticity before accepting members into the Metis Nation. 

If  you're of mixed ethnicity, you merely claim your nations, as in half Scot, half Mohawk or whatever combination, but Metis is Metis Nation.