1. You need to have "-console" in the command line options for Valheim.
2. Press "F5" ingame to open the console.
3. Type in the following commands from the console.
4. You can now press the following keys to toggle the following options:
Z - fly
B - no cost

There are also commands to spawn stuff in, but I advise you to read some of the excellent guides on developer commands available in the guides section for Valheim for those.

Ghost mode. The only critters that know you're there will be the birds.

There is also debug mode where you don't have to gather materials.

Like grin said , look up how to enable devcommands then enable builder mode, and ghost mode. Unlimited resources, no hassle. Does not work on servers as far as i'm aware (without mods) but you can build your ultimate world then move that save to a server when it's ready.