Hello newcomer, welcome!
All functions can be found in the FCN catalog (CATALOG → FCN), but since that is a menu with 43 rows, it's not necessarily the most convenient option. The other options are to use XEQ and spell out the function name using the ALPHA menu (*), or, if it's a function you think you may be using a lot, to assign it to the CUSTOM menu.
If all of this is new to you, I'd recommend taking a look at the HP-42S manual, chapter 4. There is a link to the manual in the documentation section of the Free42 web site.
(*) Or using the keyboard, if you're using the app on a computer; or using the pop-up alpha keyboard, if you're using the app on an Android or iOS device.