You have two ways with the Point of Sale to use items when checking out:

-On the bottom of your screen, you'll see a box with three dots and three lines inside it. That is your item library. Tap that and you'll get a list of all items you have created plus a couple of system items (Redeem rewards, gift cards...)

-If you're using a tablet version of Point of Sale you can create "menus" on the grid screens. Tap and hold on one of the "+" boxes until you get into the editor. You can put any item on those screens.

If you're using Square for Retail or Square for Restaurants, you can do similar steps, but the layout is a little different and I don't have the experience with them unfortunately.

If you're using Square Terminal or the Point of Sale on a phone, you don't get the grid options. You'll have to access your items through your item library (step 1 above).

More info can be found in this help file, especially the Set Up Your Item Grid section.

Hope this helps!