それも details
Examples
and at that; and not just any — adds the detail that makes the point land
昨日、三時間も待たされたんだ。それも、外で立ったまま。
kinou, sanjikan mo matasareta n da. soremo, soto de tatta mama.
I was kept waiting three whole hours yesterday. Standing outside, at that. casual
彼女は一人で山に登ったそうです。それも、冬の富士山に。
kanojo wa hitori de yama ni nobotta sou desu. soremo, fuyu no fujisan ni.
Apparently she climbed a mountain alone. Mount Fuji in winter, no less. neutral
予約が三件も重なってしまいました。それも、すべて同じ日の午後です。
yoyaku ga sanken mo kasanatte shimaimashita. soremo, subete onaji hi no gogo desu.
Three bookings ended up overlapping. All of them on the same afternoon, no less. neutral
Formation
それも、clauseattaches to plain clause
Notes
(…それも、外で立ったまま。), because it is a detail of the thing
already stated and not a new claim.
Related patterns
Similar
それに brings in a SEPARATE point about the topic (安い。それに、近い); それも goes back into the point just made and specifies it — the thing after それも is the same event or object, now pinned down (三時間待った。それも、外で) — so two unrelated facts cannot take それも.
しかも may bring in a wholly separate fact (遅れた。しかも、謝りもしなかった) as readily as it sharpens the one just stated; それも can only do the second, so it cannot stand where しかも's addition is not a specification of what came before — 謝らなかった is no detail of 遅れた.
Lookalikes
Where それ is an ARGUMENT of what follows, それも is just それ＋も — "that one too" (それも面白そうだね) — and the sentence predicates something about それ; the connective それも predicates nothing, and what comes after it is a detail of the previous sentence, not a comment on それ.
One と apart and constantly mistyped for each other, but それとも needs what follows it to be interrogative and asks the listener to pick, while それも asks nothing and merely adds a detail to a statement already made.