Сектор 25 находится вокруг булла.

• сектор is in the nominative singular because it is the subject of the sentence.
• булла is in the genitive singular.

The dictionary form is булл, but after вокруг it changes to булла.

So the structure is:

• Сектор 25 = the subject
• находится вокруг булла = the predicate, describing where it is

Because вокруг requires the genitive case.

• Nominative: булл
• Genitive: булла

This is very common in Russian: certain prepositions force a specific case, and вокруг is one of the prepositions that takes the genitive.

вокруг means around, and when it is used as a preposition before a noun, that noun goes into the genitive.

Examples:

• вокруг дома = around the house
• вокруг стола = around the table
• вокруг булла = around the bull

So if you learn вокруг, it is useful to memorize it together with the rule:

вокруг + genitive

Here находится means is located or is situated.

So this is not really the everyday verb to find in the English sense. In this context, находиться is a common Russian verb for location.

Compare:

• Книга находится на столе. = The book is on the table.
• Город находится на севере. = The city is in the north.

So in your sentence, находится is simply saying where сектор 25 is.

Sometimes yes, but it changes the feel.

Russian often omits the present-tense verb to be, especially in simple statements. So a shorter version may be possible in context. However, находится makes the sentence sound clearer and more explicit, especially in descriptions, instructions, diagrams, or formal explanations.

So:

• Сектор 25 находится вокруг булла. = full, neutral, explicit
• Сектор 25 вокруг булла. = more compressed, more context-dependent

For a learner, keeping находится is usually the safest choice.

Because Russian has no articles.

English uses a/an and the:

• the sector
• the bull

Russian does not have separate words for those. Whether something is definite or indefinite is understood from context.

So Сектор 25 находится вокруг булла can naturally correspond to English with the even though Russian does not say it directly.

Both patterns are possible, but they are used a little differently.

• сектор 25 often treats 25 as a label, number, or value
• 25-й сектор or двадцать пятый сектор means the 25th sector

So:

• сектор 25 = sector 25 / sector labeled 25
• 25-й сектор = the twenty-fifth sector

In technical contexts, games, charts, maps, and diagrams, Russian often uses noun + numeral as a label.

You read 25 as двадцать пять.

So the whole sentence aloud is:

Се́ктор два́дцать пять нахо́дится вокру́г бу́лла.

If you mean it as a label, Russian still commonly reads the number normally out loud.

No. Russian word order is fairly flexible, although the version you have is the most neutral:

Сектор 25 находится вокруг булла.

That order is good because it goes:

1. what we are talking about
2. the verb
3. the location

Other orders are possible for emphasis, but they may sound less neutral. For learners, the given order is the best one to use first.

It is a borrowing, used especially in contexts like darts. In Russian, borrowed nouns often still decline if they fit Russian grammar well, and булл is treated as a masculine noun.

Basic forms:

• Nominative: булл
• Genitive: булла
• Dative: буллу
• Accusative: булл
• Instrumental: буллом
• Prepositional: булле

So in your sentence, булла is just the expected genitive form after вокруг.