18.50The five snippets I would memorize

1. DataWeave object

%dw 2.0
output application/java
---
{
    External_Customer_ID__c: payload.customerId,
    Name: payload.companyName
}

2. DataWeave array

payload map (item) -> {
    Id__c: item.id,
    Name: item.name
}

3. SOQL

SELECT Id, Name
FROM Account
WHERE External_Customer_ID__c = :customerId

4. Choice

Choice
when condition
  → flow A
otherwise
  → flow B

5. Main flow

Listener
→ Validate
→ Transform
→ Salesforce Upsert
→ Transform Response

Everything else can be reconstructed around those.