Learn Farsi: تمام مواد باید توسط دولت بازیافت شوند. - All materials must be recycled by the government.

tavassot-e means "by" or "via" and is used to introduce the "agent" (the doer of the action) in a passive sentence. You could use be vasile-ye instead, but tavassot-e is often preferred for people or organizations (like the government), while be vasile-ye (literally "by means of") is more commonly used for tools or inanimate objects.

Why is the verb shavand instead of mishavand, and where is the typical be- prefix?

The verb follows bâyad (must), which requires the subjunctive mood. The regular present subjunctive of shodan (to become/be) is beshavand. However, in compound verbs like bâzyâft shodan (to be recycled), the be- prefix is almost always dropped, leaving just shavand.

Is mavâd a plural word? It doesn't have a standard Farsi plural ending.

Yes, mavâd is the plural of mâdde (material/substance). It is a "broken plural" borrowed from Arabic, meaning its internal word structure changes entirely instead of taking a standard Persian plural suffix like -hâ.

Since mavâd (materials) is an inanimate object, shouldn't the verb be singular?

In Farsi, plural inanimate subjects often take a singular verb. However, using a plural verb like shavand is also perfectly correct and is frequently used in formal writing or to emphasize that every single material is involved. You could also correctly say here.