1.
postgres=# SELECT postgis_full_version();
                                                                            postgis_full_version                                                           
                 
-----------------------------------------------------------------------------------------------------------------------------------------------------------
-----------------
 POSTGIS="2.3.7 r16523" PGSQL="100" GEOS="3.9.2-CAPI-1.14.3" PROJ="Rel. 4.9.3, 15 August 2016" GDAL="GDAL 1.11.4, released 2016/01/25" LIBXML="2.9.1" LIBJS
ON="0.11" RASTER
(1 row)

2.
postgres=# SELECT oid::regprocedure,probin
  FROM pg_catalog.pg_proc
  WHERE proname LIKE 'postgis_%version';
             oid              |        probin         
------------------------------+-----------------------
 postgis_version()            | $libdir/postgis-2.3
 postgis_liblwgeom_version()  | $libdir/postgis-2.3
 postgis_proj_version()       | $libdir/postgis-2.3
 postgis_lib_version()        | $libdir/postgis-2.3
 postgis_geos_version()       | $libdir/postgis-2.3
 postgis_svn_version()        | $libdir/postgis-2.3
 postgis_libxml_version()     | $libdir/postgis-2.3
 postgis_full_version()       | 
 postgis_libjson_version()    | $libdir/postgis-2.3
 postgis_raster_lib_version() | $libdir/rtpostgis-2.3
 postgis_gdal_version()       | $libdir/rtpostgis-2.3
(11 rows)

postgres=#