Cant change main layer

Im new to blender and trying to complete something on a youtube tutorial. Im trying to do a bullet through glass animation. The tutorial im using is At 18:55 when he says to edit mode and to press cntrl j the layer hes using at the bottom turns yellow instead of grey but no key pops up for him doing this. I cant figure out how he did this and i cant switch to edit mode or do cntrl j on that layer without accomplishing this though. Please help

The layer changes from grey to yellow because he selects a different object. You can see the selected object name in the bottom left corner of the viewport change from ‘Plane’ to ‘Plane_cell.522’. The yellow indicator tells you on which layer the currently active object is on.

If you cannot change to edit mode then ensure you have an object selected and and for Ctrl+J to join objects ensure you only have mesh objects selected