I start my chicks off with chick grit, and then when they are a bit older they are offered flint grit, then a few weeks before lay they have flint grit and oystershell grit on offer in separate container
I doubt if all that is really necessary, but that is what I have always done, and my thoughts are if it ain't broke don't fix it
I know another well respected breeder who has kept poultry since the 60's (and winning prizes with birds and eggs) and he says he never gives his birds any grit at all - so "you pays your money - you takes your choice"
All the best
Sue