Regarding the products, the "key" is the SKU in Megaventory and the "Product Name" in QBO and the sync is both ways:
SKUs that exist in Megaventory but do not exist in QBO as Product Names will be created in QBO
Product Names that exist in QBO but do not exist in Megaventory as SKUs will be created in Megaventory
SKUs that exist in QBO as Product Names will be left as is.
As such, it is advisable to compare the Megaventory SKU and the QuickBookes Online Product Name fields in both apps to make sure they match (if it's intended for the products to match).
Regarding Suppliers and Clients, the Supplier Client Name is the key in both apps. The functionality is the same as above with Products.
Regarding the inventory levels / product stock, you should make sure that the Megaventory inventory levels match the on-hand quantity of QuickBooks before connecting the two apps. To update the inventory levels in Megaventory, you can do an inventory adjustment operation which will update the inventory levels to match these of QuickBooks Online and then, connect the two apps.