Learn Turkish: Milletçe tonlarca atığı geri dönüşüme gönderdik. - As a nation, we sent tons of waste to recycling.

The suffix -ce / -ça (which becomes -çe here due to consonant harmony) attaches to a noun to show a collective action. Millet means nation, so milletçe translates to as a nation or all together as a nation. It emphasizes that the action was done collectively by the entire group.

Both milletçe and tonlarca use the same adverbial suffix. Why does tonlarca have the plural -lar while milletçe does not?

They use the same suffix for two different functions taught in this lesson. In tonlarca (tons of), the plural -lar combined with -ca expresses an extensive, exaggerated quantity. For milletçe, the root millet is already a collective noun. If you added the plural to make milletlerce, it would mean by many nations, whereas here the meaning is as one single, collective nation.

Why does atık (waste) change to atığı?

Atık takes the specific accusative suffix -ı because it is the direct object of the verb gönderdik (we sent). Even though we are talking about a massive quantity, the sentence treats this collected waste as a specific, defined entity. As usual, adding a vowel suffix to a word ending in k causes it to soften to ğ.