NodeCore only supports "survival" gameplay (i.e. sandbox play with constrained resources) natively. Alternative game modes require some mods:

Creative Mode

Build/dig and maneuver freely, to build artistic creations or prototype complex machinery.

Grant yourself all permissions. With fly and noclip you can move about freely, with ncdqd you are immune to damage, with give you can give yourself any material, and with server and debug you can override some of the mechanics of the game.
Use the /stasis chat command to toggle stasis mode. When the world is frozen in stasis, most time-based mechanics (except flowing liquids) are frozen, e.g. fire will neither spread nor extinguish, circuits won't tick, etc. You can default the game to starting in stasis mode by setting nodecore_stasis = true in your minetest.conf
The szutil_givemenu mod from SzUtilPack (https://content.minetest.net/packages/Warr1024/szutilpack/) makes it a lot easier to search for items with the /givemenu command. With the pulverize priv, you can /pulverize or /clearinv to destroy excess items.
You can dig any node with the special admin hand tool, which you can get via /nckfa (or via /give or /givemenu). It acts as a maximum-tier tool for every dig group. If you aux+sneak+punch with it, it unconditionally digs the pointed node and places it in your inventory, even for nodes that are not diggable.

Adventure/Puzzle Mode

Play a pre-built map where you cannot build/dig freely and must interact within the constraints of the puzzle.

You can create adventure/puzzle maps using the puzzlemap mod on ContentDB (https://content.minetest.net/packages/Warr1024/puzzlemap/). More detailed instructions are on the CDB page / mod README.