The collectors guide says one of the options is not using any faction. The only way I can unlock the special platform to build is by talking to a faction.

Any help would be appreciated, I still haven't decided who I want to join.

Building the relay with a faction (Minutemen, Railroad or BoS) doesn't lock you into that faction. The BoS was a bit miffed about me building it at the Castle, but offered me all their post-Institutionalized quests regardless. The Railroad didn't even comment on it.

It will be made very clear, when you finally decide against factions.

Appreciate the advice. Just saying it is listed as an option to build it by yourself in the official game guide. Can't build it by myself though haha. Yeah I used the minutemen just to get it done

I believe you can build with any faction and you won't get locked out, for instance if you build with the Brotherhood and visit the Railroad Des just happens to hear a rumour that you built the relay with the BOS and asks you to do the job they would have asked you to do if you had built the relay with them.

At least that is what happened for me.