Snapshot and Dynamic are properties assigned to skills and bursts that last over time. Most summons in the game snapshot, dynamic skills are the exception, and if they aren't listed in the Dynamic Skills section, they can be assumed to snapshot.
Summons that snapshot use the character's stats when cast, and will continue to use those stats regardless of buffs gained or lost in that timeframe. Only the stats visible on a character's detailed attributes page get snapshotted, buffs that affect invisible stats, such as 4pc Blizzard Strayer or Solar Pearl's passive, still effect the damage output of a snapshotting summon.
Dynamic summons continuously update based on the character's stats in real time.
Transformative Reactions do not snapshot EM as of patch 1.4. Even if a skill snapshots, their EM will update real time. Transformative reactions include: overload, superconduct, electro-charged, shatter, swirl, and crystallize.
Fischl summons Oz outside of Bennett's burst. Oz deals 920 electro damage and 1870 on overloads. Sucrose shares EM with Fischl and Fischl steps into Bennett's burst. Oz still deals 920 electro damage, but overload damage has been increased to 1900.
Chongyun's Spirit Blade: Cloud-Parting Star
Eula's Glacial Illumination
Hu Tao's Blood Blossom
Keqing's Starward Sword
Mona's Stellaris Phantasm
Xingqiu's Guhua Sword: Raincutter