Instead of hardcoding Combo box , we can bind it to the Choice / Option Set field of Dynamics 365 / Dataverse.
Format – Choices(DataSource[@columnname])
Check out the post that explains it – https://debajmecrm.com/how-to-bind-a-multiselect-choice-of-dataverse-to-a-combobox-in-canvas-apps/
Also check out – how to bind and do filter on multiselect combo box – https://youtu.be/5dSk5iOgT68?t=286
To learn about all the properties of Combo box refer – https://www.spguides.com/powerapps-combobox-control/
Hope it helps..