How can we get 1st index elements of each array in different new array

Appian doesn't really understand list of list and therefore it will flatten them into one big array. You might have to cast it in a dictionary so it can iterate on that. 

a!localVariables(
  local!array:{
    {
      list: {2,13,65}
    },
    {
      list: {7,15,45}
    },
    {
      list: {14,3,56}
    }
    
  },
  a!forEach(
    items: local!array,
    expression: fv!item.list[1]
  )
)

You can change the value of 1 to your desired index. 

Appian doesn't really understand list of list and therefore it will flatten them into one big array. You might have to cast it in a dictionary so it can iterate on that. 

a!localVariables(
  local!array:{
    {
      list: {2,13,65}
    },
    {
      list: {7,15,45}
    },
    {
      list: {14,3,56}
    }
    
  },
  a!forEach(
    items: local!array,
    expression: fv!item.list[1]
  )
)

You can change the value of 1 to your desired index.