Collections — the part that is genuinely new

No video for this one — collections are better learned in a worksheet than watched, so this lesson is the reading version, and it is the longest page in the course.

The three shapes

A List is a Flow record collection: ordered, allows duplicates, and you loop over it.

A Set is the collection you always wanted in Flow: it refuses duplicates, and asking “is this Id in there?” is instant. Half of bulkification is “put the Ids in a Set”.

A Map is the one with no Flow equivalent, and the one that matters most: a lookup table from key to value. “Give me the Account for this Id, without querying again” is a Map. So is “group these Contacts by their AccountId”.

The idiom to memorise

Map<Id, Account> accountsById = new Map<Id, Account>(
    [SELECT Id, Name, Type FROM Account WHERE Id IN :accountIds]
);

One query, then free lookups forever after. When you can read that line without slowing down, you read most production Apex without slowing down — it is the single most common three-line pattern on the platform.