I don't understand the following sentence:
やさい や くだもの も おおい です。
I know that や is used for incomplete enumeration and も means "also". But how to interpret such a combination of these two particles?
Your understanding of the particles as used are correct.
Depending on context, it seems like this sentence could be a direct response to another.
Original statement
肉{にく}や魚{さかな}も多い{おおい}です。
There are a lot of meat and fish.
Response statement
野菜{やさい}や果物{くだもの}も多{おお}いです
There are also many vegetables and fruits.