Field Validation Rules - Based on another field in collection

Is it possible to create a validation rule that looks at the value of another field

E.g. collection has a total_qty field and available_qty, want validation rule that checks when available_qty greater than or equal to total_qty .

Or do I have to use a flow for that?

We do not currently support looking up relational field values for Field Validation rules.

You could certainly use a Flow for this or a custom Hook if you prefer to express the logic in code.