Buildings:
• Bridge
• Safe House
• Timefall Shelter
• Postbox
• Watchtower
• Generator
• Zip-line
Fabricator:
Climbing Anchor (must then plant and use one in the outside world)
Ladder (must place one in the outside world)
Co-op build:
Roadway (just one for the trophy)
Signs:
Hold L1 to bring up the compass and then hit triangle. You must build one of EVERY sign for the trophy. These cost nothing to make so choose a spot away from where people may not find them and spam build them.
You could also be helpful and build them to warn other players or aid them, this is likely to generate likes.
EDIT: Since it has now been confirmed by two gamers, I'll add that there may have been a patch since I unlicked this. Building a few unique signs will still unlock the trophy.
EDIT 2: Sundowner1 has pointed out that since a recent patch, only one sign is now required for the trophy to pop