It seems readfile is from an extension but present in cli. json_each is present in standard sqlite core from 3.38.0 as json1 extension is also now part of core.

json_each : https://www.sqlite.org/json1.html#jeach

readfile : https://sqlite.org/cli.html#file_i_o_functions

> Note that the readfile(X) and writefile(X,Y) functions are extension functions and are not built into the core SQLite library. These routines are available as a loadable extension in the ext/misc/fileio.c source file in the SQLite source code repositories.