der Belag
topping
A layer that is placed on something, especially on foods such as bread or pizza.
Ich hätte gern ein Brötchen mit Käse als Belag.
I’d like a roll with cheese as the topping.

topping / covering
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topping
A layer that is placed on something, especially on foods such as bread or pizza.
Ich hätte gern ein Brötchen mit Käse als Belag.
I’d like a roll with cheese as the topping.

The noun 'der Belag' (topping/covering) is derived directly from the verb 'belegen' (to cover, to put something on). 'Belegen' means 'to cover a surface.' So the 'Belag' is the material that covers something.
Bitte belege die Brote mit Käse und Schinken.
Please top the sandwiches with cheese and ham.
The meaning of 'Belag' (topping/covering) is almost always clear from context. If people are talking about food, it means a topping. If it is about roads, it means the surface. In a medical context, it means a layer on the body.
Was für einen Belag möchtest du auf deiner Pizza?
What kind of topping would you like on your pizza?
In a culinary context, typical combinations are 'Belag für Pizza' (topping for pizza), 'Belag auf dem Brot' (topping on the bread), or 'süßer/herzhafter Belag' (sweet/savory topping).
Als Belag für das Sandwich wähle ich Lachs und Frischkäse.
For the sandwich topping, I’ll choose salmon and cream cheese.
When talking about surfaces, people often say 'Straßenbelag' (road surface), 'Fahrbahnbelag' (roadway surface), or 'Bodenbelag' (floor covering). A typical verb is 'den Belag erneuern' (to renew/replace the surface).
Der alte Teppich wird durch einen neuen Bodenbelag aus Holz ersetzt.
The old carpet is being replaced with a new wooden floor covering.
film
'der Film' and 'der Belag' can both mean a thin layer on a surface. 'der Film' often sounds especially thin and even, for example with grease, dust, or liquid. 'der Belag' is used more broadly and often means a visible or noticeable layer, for example on bread, a road, teeth, or the tongue. The words can be confusing because both cover something. In practice: for a very thin, almost fine layer, 'Film' often fits. For food, floors, roads, or medical layers, 'Belag' is more natural.

Nach dem Essen hatte Max einen unangenehmen Belag auf den Zähnen.
After eating, Max had an unpleasant coating on his teeth.
Auf der kalten Suppe liegt ein dünner Film aus Fett.
There is a thin film of fat on the cold soup.