SVG icons in Twenty Nineteen

You shouldn’t need to change the viewbox attribute. You can probably just use it.

The way I think about is SVGs are like they are drawn on graph paper. The viewbox sets out how many squares there are on the paper and the the path in your examples above are the guides to where to draw on the graph paper.

The top left is 0,0 and the bottom right is the extremes of the viewbox. E.g. 24,24 or 488,512.

You should be able to just swap the SVGs out and style with CSS.