It can feel complicated and sort of overwhelming. There are always new things to learn, but I think that is part of fun. Since you have the grit you might as well put it out and see how it goes but with a nice big yard with lots of dirt chances are you don't have to buy it if you don't want to bother. Of course there is no harm in leaving a bowl out in case they want it.
If you are feeding a layer feed to your flock and are getting nice egg shells then you do not have to buy any extra calcium or oyster shell.
There a number of schools of thought on all this and a lot of it has more to do with your personal goals and flock rather than a right and wrong way to go about this. If you want to keep it as simple as possible then access to dirt, a constant supply of clean water and a layer feed (for adult hens only) is all you need. Dirt will have grit naturally. Layer has extra calcium and is made to be complete nutrition.
Some people feed a layer and still keep a separate bowl of oyster as a just in case. Just like the grit, it doesn't hurt as long as it is separate from the feed. You can give scratch grains as a snack if you want also. You can take it a step further if you feel like it and grow fodder or sprout the grains.
Instead of feeding a layer, some people (myself included) feed a grower or flock raiser feed. In this case the oyster shell is a must because the feed does not have the extra calcium mixed in like the layer does. The reasons are generally if for any reason you have a flock that has non-laying or infrequently laying birds (young birds, roos, molting birds, etc). This way the birds can regulate their own calcium and the non laying birds don't eat too much, which is especially bad for chicks and very young pullets.
You can get a little tricky with it and do layer when the birds are laying a lot and then grower + oyster through the molting and winter season when the laying is less. Depending on your weather, grit may be more necessary in the winter if the birds have to be shut in but not needed in the summer.
You can get really into it and start dealing with organics and different protein types, corn or no corn -- on and on -- but again you don't really have to either. I know my system is certainly more complicated that it needs to be for my hens to do well but I have my own reasons doing what I do and I like this aspect of chicken raising a lot so I enjoy getting into these details.
Sorry, lol, this may have been more than you wanted.