Built-in scripts are indicators or strategies written by TradingView; they are not published in the Community Scripts. You can load built-in scripts on your chart via the "Technicals" group in the left pane selection of the "Indicators, Metrics & Strategies" dialog box available from the button at the top of your chart.
Most built-in scripts use our Pine Script programming language, but not all. For example, our Volume Profiles and Chart Pattern scripts do not use Pine Script; their source code is thus unavailable.
There are two ways you can access the source code of built-in indicators.
Method 1
In the Pine Editor, use the "Open" button at the top right and choose "Built-in script..." to view the list of built-in scripts written in Pine. When you select one of them, its source code will load in the Editor. From there, you can save a copy of the code as a personal script. Changes you make to that script will not affect the original TradingView built-in script.
Method 2
When you have a built-in script already loaded on the chart, bring up its pop-up menu by hovering over the script's name and clicking on its "Source code" icon ("{ }") when one exists. If none exists, the built-in does not use Pine Script, and its source code is unavailable.
When the built-in's source code is available, it will appear in the Editor with a gray background indicating that it is read-only. From there, you can make a copy of it by bringing up the Editor's "More" menu (the three dots at the top right) and selecting "Save as...". You will then be able to adapt that copy however you see fit.