No-one is 100% sure.
But the Indians are not a single people and did not all come over at the same time or from the same place.
The Baring Strait Land Bridge existed between 25k & 14 years ago. People arrived in Siberia around 25k years ago and settled widely forming different cultures. No-one really knows when the migration began, it would however have grown in numbers as the people in Siberia did. The height of the Siberian culture was between 18k ya & 15 ya.
This shouldn't be seen a mass migration, but small groups of hunters following the animals. Some would have crossed back and contact would have been maintained, in fact technology was passing back & forth.
The land bridge also existed between 50k years ago & 40k years ago and there have been finds of tools across the Americas pre-dating dating 25,000 years ago and up to between 50k and 40k years ago, and now several bodies have turned up. So there is a growing body of evidence a previous people already lived in America when the Indians arrived.
The initial Indian migrants settled in Siberia but began to make their way down the west coast into the west central USA, west coast and Northern Mexico. They were very small in numbers but found the rich hunting on the plains and mega fauna so are believe to have thrived and expanded numbers quickly. Later as the beasts ran out they moved further south and east in search of more hunting.