Can I make a mod that lets me build structures with just a chat command?

Try Tynker's Mods Editor (Project > Create Project > Minecraft > Create Mod).

https://www.tynker.com/dashboard/student/#/minecraft/mod/bedrock

Then experiment with Tynkerbot's building commands:

Tynkerbot can place blocks individually. And that can be a great way to create truly advanced structures. But first, check out Utilities, they’re an easy way to build house shapes quickly using Tynkerbot. Here’s a command to build an empty box made of bedrock blocks.

Here are some ideas:

• Use a hemisphere (half a sphere) to build an igloo using snow — or a Thunderdome-style fighting cage using fences!
• Try prisms, lines, stairs, and other shapes, too!

Read this article for more details: "Create a Minecraft House with Tynker Code."