Regional Features
Updating and refining boundaries between adjacent GeoJSONs is not too hard. It is recommended that users wishing to perform such edits use QGIS. While QGIS 3.20 is pictured below, the tools used have been in the application for many versions, and users who opt for the LTR will be as able to perform these edits as those on the latest release.
Open a new project in QGIS. Browse to the repository in the Browser pane and find the features you wish to edit.
From there, you can simply drag the features into the map canvas. When adjusting a boundary, you must load all features which participate in that boundary. This will typically be between two or maybe three areas. While not strictly necessary, you may load all the features.
This is what ensures your edits do not create overlaps or gaps, so it's very important. On the Snapping Toolbar, click the magnet icon to enable snapping. The toolbar may not be visible at first; go to View > Toolbars > Snapping Toolbar from the menu bar, or right-click the toolbar area and select the snapping toolbar from the context menu.
On the toolbar, you will see an icon with three ways joined by a shared node:
Click this icon to enable topological editing.
In QGIS, there is an Editing Toolbar. (Look for the pencil icon.) With a layer selected in the Layers pane, click the pencil icon for Toggle Editing. You can also right-click the layer and find this command in the context menu.
NOTE: You will need to repeat this for each layer being editing. If I adjust the IL / IN boundary with editing only enabled on IL, I will only affect the IL GeoJSON.
Near the Toggle Editing icon, you'll find the Vertex Editor icon, a hammer and screwdriver under a way/node. Click this to activate the Vertex Editor tool. The cursor will change to a crosshair, and as you hover over edges and nodes, they will highlight.
Click on a vertex to "pick it up", click again to "put it down". The same is true for edges and selections of multiple nodes, though in this context that is probably less useful. On the midpoint of any edge, a "helper node" is visible as a grey +. Clicking this will add a new vertex at that point, which can be dragged into the desired position.
When there are pending edits, the Save icon will be enabled on the toolbar. Simply click the icon to save your edits to the layer.
NOTE: As with Toggle Editing, this button only applies to the currently selected layer. It's possible to successfully edit two layers and inadvertently only save the edits to a single layer. Fortunately, the Edit Toolbar does have a quicker way to save multiple edits. To the left of Toggle Editing is the Current Edits icon, two red pencils. Click this icon to open a small dropdown menu, where you can find the Save for All Layers command.
Once you've saved your edits, you may commit the changes. If possible, run npm build first, though someone else can do this after the fact as well.
Here are a few things that might make your life easier using QGIS.
For several versions, QGIS has come with the OSM carto tiles as a pre-configured XYZ Tile service, which you can find in the Browser pane. Drag this into your map to help guide your edits. You can also bring in aerial imagery, vector layers, etc.. Vector layers have the benefit of being "snappable".
By default, your vector layers come in as opaque areas. To make your reference layers actually useful, put them higher up on the layer list. For a raster layer like the OSM tiles, you can set the Blending Mode to something like multiply in order to view the layers below.
You can add layers to groups in the Layers pane. This makes it easier to keep different regions, reference layers, etc., separate, and keeps the layer list from getting too long.
Load all the features from the NSI repo and save your QGIS project. This will help save you setup time if you need to adjust other boundaries in the future.