Here are quick notes from what I learned from GSI Maps Vector style.json.

Sources

• Vector tiles are from https://maps.gsi.go.jp/xyz/experimental_bvmap/{z}/{x}/{y}.pbf
• Font data (glyph) are from https://maps.gsi.go.jp/xyz/noto-jp/{fontstack}/{range}.pbf, which means they are using noto-jp.
• From separate research, the icons (sprite) are from https://maps.gsi.go.jp/vector/sprite/std.json and https://maps.gsi.go.jp/vector/sprite/std.png

Fonts used

NotoSansCJKjp-Regular. Because they are using a relatively old version of the Mapbox GL JS, the font data are downloaded and then rendered.

Layers

A lot of layers are there.

Style

They are not using modern expressions. With modern expressions, you can rewrite filters like

filter: [
  "all",
  [
    "==",
    "ftCode",
    7541
  ]
]

into

filter: [
  "match",
  [
    "get",
    "ftCode"
  ]
  [
    7541
  ]
  true
  false
]