This directory is only for providing custom portals. At the moment, you only may set a custom portal when entering the nether.
To do so, call your file "to_nether.nbt" or "to_nether.snbt".
It needs to contain at least one nether portal block. If that is destroyed, the structure would re-generate when re-entering the nether.
To place additional structures around the portal on a team's first nether visit, use the "netherSpreads" config in templates.json5.
Spread files are saved to the spreads directory (not this one) using the structure saver item in spread mode.