[orphean]

Later on assemblers like Merlin and ORCA/M added the ability to assemble source files as separate objects which were then linked.

Earlier assemblers (and the later ones!) also typically supported a capability to reference another source file from another which lets a large assembly chain itself without needing to have the whole thing in memory at once.

At runtime, you could use overlays to essentially load in program modules in and out of RAM dynamically so you'd be surprised at the (relative) sophistication of some software packages back then!