There are several different API calls you can use to query products, but exactly which to use and when will really depend on the context of what information you need and what you already have to work with.
Some general notes about querying products
Sometimes, the same function exists on different endpoints as the endpoint can be used to refine the query by allowing for more inputs.
getProductByManufacturerSKU is available on both the Products and Sales Channel endpoints, however - only a SKU can be given in the context of the product, however to check the same product in context of the Sales Channel, you would be expected to also provide a
Be aware, that these functions are likely to return the same standard Product objects - namely:-
a "lite" version, which has minimal data but will provide enough to find products that match what you are looking for and give you the IDs you need to do a more detailed lookup
a "full" version, which has a placeholder for everything, however, depending on the context the function is used and system configuration, not all values will be necessarily be populated
More detailed Product information via API guides
getProductByManufacturerSKU- gets the detailed information about a product given a particular SKU