Modular buildings are one of the more unique aspects of Foundry. They are large scale structures that have a variety of utility when it comes to progression or just looking really good in your factories!
To get started with modular buildings the first thing you need to do is actually research them, the tech can be found just before the Science Pack IV research in the tree and includes everything you need in order start construction.

The modular building planner can be found in the handheld section of your crafting menu and it's how you will assign modules to your building like shown below. Selecting each of these red nodes will bring up a drop down where you can select that module to add but as of right now there isn't a lot of choice when it comes to modular buildings and you will either max out the building or partially build it but you never have to choose between modules for each slot. As previously mentioned each modular building can have a max amount of each module and that is shown on the top left box with a min and max value, the min value is the minimum number of this module type for you to be able to hit confirm and start construction of the building.

I recommend your first modular building to be the Radio Tower as this will be important for obtaining firmarlite bars which are vital for progression however the radio tower itself has a time sink associated with it when repairing the space station in orbit so it's better to do this sooner to get a head start on those upgrades. I wrote more information regarding this on the wiki entry for those who are stuck with this step. See More[wiki.foundry-game.com]

Additionally modular buildings aren't free, each module that you add costs construction materials and these construction materials need to be crafting inside an assembler and loaded into a construction warehouse or they cannot be access by the drone ports.

Each construction warehouse can store a maximum of 25,000 construction materials but you can have multiple warehouses if you find that you need a larger supply of materials for larger projects.

There are 2 types of ship ports used in the construction of modular buildings, the transport ship port and the construction ship port. Transport ship ports have a number of uses but in this specific case their job is to transport construction materials from your construction warehouse to the modular building site. Each trip carries at most 250 construction materials so for longer trips to further away bases you may want to increase the quantity of transport ship ports you have.

Once those construction materials are delivered the construction ship port will begin progress on building the structure (The progress is shown as a percentage on the bottom left of the UI if you left click the modular structure's base again) once that finishes you should have a fully functional structure!

Be sure to connect your new structure to your high voltage grid at the connection point that was set when you configured the structure if necessary and the same applies to both drone ports! a high voltage connection is needed for both transport and construction ship ports to function