Sacituzumab Govitecan in Metastatic Salivary Gland and Thyroid Cancers (SG-SGTC)

The goal of this clinical trial is to learn if the drug, sacituzumab govitecan, works to treat: 1. Salivary gland cancers that have spread to other parts of the body or come back after previous treatment, and cannot be removed by surgery 2. A type of thyroid cancer (specifically, differentiated thyroid carcinoma) that has spread to other parts of the body or come back after previous treatment, no longer responds to radioactive iodine treatment and cannot be removed by surgery The main question it aims to answer is whether sacituzumab govitecan can treat the 2 conditions mentioned above in a safe and effective manner. Participants will: * Receive sacituzumab govitecan by intravenous (IV) infusion on Day 1 and Day 8 of each treatment cycle. Each treatment cycle is 21 days (3 weeks) long. * Visit the clinic before each infusion for checkups and blood tests * Go through CT/PET/MRI scan once every 6 weeks for the first 24 weeks, then every 9 weeks thereafter. * Go through 2 tumour biopsies (if assessed by the doctor to be safe) at the start and at the end of study participation Participants will continue to receive treatment with sacituzumab govitecan until their cancer stops responding to the drug or if they decide to withdraw from the study.