Вынос границ начали утром.

Here вынос is a noun, not a verb.

It is a verbal noun built from выносить / вынести and names an activity or process. In this sentence, вынос границ is the name of the work being started.

So the structure is roughly:

• начали = they started
• вынос границ = the boundary-marking / boundary staking-out work

Because границ depends on the noun вынос.

In Russian, action nouns often take the thing affected in the genitive:

• чтение книги = reading of a book
• строительство дома = construction of a house
• вынос границ = setting out of boundaries

So границ is not directly controlled by начали. It belongs with вынос as part of one noun phrase.

Also, границ is the genitive plural of граница.

Because вынос границ is not the subject here. It is the thing that was started.

The subject is omitted, but understood as an indefinite they:

• начали = they started

Russian very often uses 3rd person plural this way when the people doing the action are unknown, obvious, or simply unimportant.

So the sentence means something like:

• They started the boundary work in the morning.

The main clue is the verb form.

If вынос were the subject, the verb would normally agree with it and be singular, for example:

• Вынос границ начался утром. = The boundary work began in the morning.

But your sentence has начали, which is plural. That means there is an implied plural subject, and вынос границ is what they started.

So:

• Вынос границ начали утром. = They started the boundary work in the morning.
• Вынос границ начался утром. = The boundary work began in the morning.

Both are possible, but they are structured differently.

Both can be correct, but they say slightly different things.

• начали focuses on the people who started the work, even though they are not named.
• начался presents the event more by itself: the work began.

So the difference is:

• Вынос границ начали утром. = They started the boundary work in the morning.
• Вынос границ начался утром. = The boundary work began in the morning.

English often makes these sound very similar, but Russian gives you a choice of perspective.

Начали is the past plural of начать, which is perfective.

That is natural here because the sentence refers to a completed starting point: the start happened in the morning.

• начали = they started
• начинали = they were starting / used to start / started repeatedly, depending on context

With a single completed event, начали is the normal choice.

Утром is the instrumental singular of утро, and Russian often uses this form adverbially to mean in the morning.

This is a very common pattern:

• утром = in the morning
• днём = in the daytime
• вечером = in the evening
• ночью = at night

So утром here is just a time expression.

Yes, Russian word order is flexible.

This order puts вынос границ first, which makes that activity the topic or focus of attention. It sounds like the speaker is introducing or highlighting exactly what was started.

Other orders are possible, for example:

• Утром начали вынос границ.
• Начали утром вынос границ.

These can all be grammatical, but the emphasis changes.

So the original order is not random: it helps organize the information.

The normal stress is:

Вы́нос грани́ц нача́ли у́тром.

That gives you:

• вы́нос
• грани́ц
• нача́ли
• у́тром

Stress matters in Russian, so it is good to learn each word together with its stress pattern.