By default, the "clip tray" is just a single-item buffer, with that item being replaced every time you select something new and tap "copy" or "cut". Some devices (Samsung used to do this) included a clip tray manager out of the box that made that buffer deeper.
Bottom line - if you phone doesn't have one of those apps, you access the "clip tray" by long-pressing in a text entry area and tapping "Paste" on the pop-up menu. If you want a deeper buffer (I highly suggest NOT using the clip tray as a long-term storage area) you can search the Play Store for apps to do that.
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