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Settings → Appearance → Typography covers the everyday choices; the rest is config.json.

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Hack is bundled

The default font ships inside the binary. It renders identically on every machine without relying on a system install, so a fresh laptop looks like the one you set up last year.

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Fallbacks

font_family is the primary face; font_fallbacks is an ordered list tried in turn for anything the primary lacks.
Leave font_fallbacks out and you get the platform’s default chain: Each ends in faces the host OS actually ships, and those stock names are appended to whatever list you write yourself — so a config.json copied from another platform still resolves.

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OpenType features

font_features passes tags straight through to the shaper:
A tag must be four alphanumeric characters; true/false map to 1/0. Anything malformed is skipped with a log line rather than failing the whole config.

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CJK and the two-column grid

A cell is one advance of the primary face, and a wide (CJK) character is pinned to exactly two of them. A CJK fallback sits flush in its slot only if its ideographs advance twice the primary’s Latin advance.
Bundled Hack advances 0.60205em, so a two-column slot is 1.2041em — while every stock CJK face (Microsoft YaHei, PingFang SC, Noto Sans CJK) advances 1.0em. Those glyphs get left-aligned in the slot, leaving a ~0.2em gap on the right of every character. Maple Mono NF CN is in every platform’s default chain for exactly this reason: 0.6em Latin, 1.2em CJK, an exact two-cell fit against Hack. It leads the chain on Windows and Linux, and on macOS sits behind Menlo and Hasklug, which cover Latin and Nerd Font glyphs first. It is referenced by name only, never bundled (~20 MB per weight) — install it and tty7 picks it up with no config change. If you want CJK set tight rather than merely even, change the primary face instead. One that advances 0.5em — Sarasa Mono SC, say — makes two columns exactly 1.0em.