У опушки находится грибница.

Because опушки is the genitive singular form of опушка.

The preposition у usually requires the genitive case when it means by, near, at, next to.

• dictionary form: опушка
• after у: у опушки

So у опушки means near the forest edge or by the forest edge.

Here у means by, near, or at.

It does not mean at in the sense of being directly on top of something, and it does not mean possession here. Russian у can also be used in sentences like У меня есть... (I have...), but that is a different use.

In У опушки находится грибница, it is purely a location word:

• у опушки = near the forest edge

Находится means is located, is situated, or is found.

Russian often leaves out to be in the present tense, but with location, находится is a common way to sound more explicit or slightly more formal/descriptive.

Compare:

• Грибница у опушки. = The mycelium is by the forest edge.
  This is shorter and more neutral/conversational.
• Грибница находится у опушки. = The mycelium is located by the forest edge.
  This sounds more descriptive or matter-of-fact.

So находится adds the idea of being situated somewhere.

Because the subject is грибница, which is singular.

Even though грибница comes after the verb, it is still the grammatical subject:

• грибница = singular
• therefore находится = singular

If the subject were plural, the verb would also be plural:

• У опушки находятся грибы. = There are mushrooms by the forest edge.

Russian word order is much more flexible than English word order.

Starting with У опушки puts the location first. This often happens when the speaker wants to set the scene first and then say what is there.

So:

• У опушки находится грибница. = emphasis first on where
• Грибница находится у опушки. = emphasis first on what

Both are grammatical. The meaning is basically the same, but the focus is a little different.

Yes, absolutely.

That version is very natural too. It simply uses a more straightforward subject + verb + location order.

• У опушки находится грибница.
• Грибница находится у опушки.

Both mean the same thing. The first one sounds a bit more like At the forest edge, there is a mycelium / By the forest edge is a mycelium, with the location introduced first.

Грибница is in the nominative case because it is the subject of the sentence.

The basic form is:

• грибница = nominative singular

It is the thing that is located somewhere, so it controls the verb agreement.

Опушка usually means the edge of a forest or the woodland edge.

So у опушки suggests a place near the edge of the woods, not just any random edge.

This is one of those words whose natural English translation depends on context:

• by the forest edge
• near the edge of the woods
• sometimes at the edge of the forest

No. Russian does not have articles like a/an and the.

So грибница could be understood as:

• a mycelium
• the mycelium

Which one sounds best depends on the context you already have.

The same is true for опушки:

• near a forest edge
• near the forest edge

Russian leaves that distinction to context.

A standard stressed version is:

У опу́шки нахо́дится грибни́ца.

Stress:

• у опу́шки
• нахо́дится
• грибни́ца

This can help because Russian stress is not always predictable and can affect pronunciation a lot.

Because находится is the conjugated verb form, while находиться is the infinitive.

• находиться = to be located
• находится = is located

This is a very common contrast in Russian:

• infinitive: делать = to do
• 3rd person singular: делает = does

So in the sentence, you need the finite verb:

• грибница находится

It can sound a little more formal or descriptive than simply leaving the verb out, but it is still perfectly normal Russian.

It is especially common in:

• descriptions
• explanations
• textbooks
• factual statements
• maps or location-based language

So it is not strange at all here. It just has a slightly more explicit is located feeling than the simpler Грибница у опушки.