Hey @DaMask,
When looking to retrieve associations of deals to company/contact, you would need to use the CRM API | Associations.
Specifically the. POST crm/v3/associations/{fromObjectType}/{toObjectType}/batch/read
i.e. crm/v3/associations/deals/contacts/batch/read
{
"inputs": [
{
"id": "{{deal id}}"
}
]
}
The return response will include the associated contact IDs.