deetasiyouryou ni kakawarazu, kihonryoukin ha onazi desu.

にかかわらず means regardless of, irrespective of, or whether or not X changes.

So:

• データ使用量にかかわらず = regardless of data usage
• Full sentence: Regardless of data usage, the basic fee is the same.

This is a set expression, so it is best learned as one grammar pattern:

• Noun + にかかわらず
• Clause/dictionary form + かかわらず in some cases

It is commonly used in formal or written Japanese.

Because にかかわらず is a fixed grammar pattern.

In this sentence, データ使用量 is a noun, and nouns connect to this pattern with に:

• データ使用量 + にかかわらず

So here, に is not being chosen freely the way it sometimes is with other grammar. It is simply part of the expression N にかかわらず.

You should think of it as one chunk:

• 年齢にかかわらず = regardless of age
• 天候にかかわらず = regardless of the weather
• データ使用量にかかわらず = regardless of data usage

Yes. かかわらず comes from 関わる.

Very roughly, 関わる means to be related to, to concern, or to have to do with. In the grammar pattern にかかわらず, the overall meaning becomes without being affected by X or regardless of X.

You will often see it written in hiragana as かかわらず, even though it is related to the kanji 関わる. Writing it in hiragana is very common for grammar expressions like this.

Literally, it is something like:

• 基本料金 = base fee / basic charge
• は = as for
• 同じ = the same
• です = polite copula

So the structure is:

• As for the basic fee, it is the same.

In more natural English, that becomes:

• The basic fee is the same.

は marks 基本料金 as the topic of the sentence.

The sentence is saying:

• As for the basic fee, regardless of data usage, it stays the same.

Using は makes sense because the speaker is presenting 基本料金 as the thing being talked about.

If you are translating literally, it can help to think:

• 基本料金は = as for the basic fee

In natural English, though, we usually just say:

• the basic fee

Yes, but it behaves a little differently from the standard -i adjectives and -na adjectives.

同じ means same, and in a sentence like this it works with です:

• 同じです = is the same

It can also directly modify a noun:

• 同じ料金 = the same fee
• 同じ会社 = the same company

Many learners notice that 同じ does not fit perfectly into the usual adjective categories they first learn. That is normal. The important thing for now is:

• 同じです = is the same
• 同じ + noun = same + noun

The comma helps separate the introductory phrase from the main statement.

So the sentence is divided like this:

• データ使用量にかかわらず、 = regardless of data usage,
• 基本料金は同じです。 = the basic fee is the same.

Japanese commas are often used more flexibly than English commas, but here it makes the sentence easier to read and shows a clear pause.

Yes, にかかわらず sounds fairly formal and is common in:

• announcements
• contracts
• advertisements
• customer service language
• written explanations

For everyday conversation, people might use other expressions depending on the situation, but にかかわらず is very natural in things like pricing, rules, and official statements.

So this sentence sounds like something you might see on:

• a phone plan description
• a website
• a service contract
• a bill or pricing page

Yes, very often. It is a productive pattern.

Examples:

• 年齢にかかわらず、応募できます。
  You can apply regardless of age.

• 経験にかかわらず、歓迎します。
  Applicants are welcome regardless of experience.

• 天候にかかわらず、試合は行われます。
  The game will be held regardless of the weather.

So once you learn the pattern noun + にかかわらず, you can use it in many situations.

Both can express an idea like whether X changes or not, but they work differently.

にかかわらず:

• more formal
• concise
• often used with nouns
• has a written or official feel

Example:

• データ使用量にかかわらず、基本料金は同じです。
  Regardless of data usage, the basic fee is the same.

A ても style expression:

• often more conversational
• can sound more direct or specific
• often explicitly gives alternatives

Example:

• データをたくさん使っても、あまり使わなくても、基本料金は同じです。
  Whether you use a lot of data or not much, the basic fee is the same.

So the original sentence is more compact and more formal.

The structure is:

• [Condition/thing that does not matter] にかかわらず、[main statement]

In this sentence:

• データ使用量にかかわらず、 = regardless of data usage,
• 基本料金は同じです。 = the basic fee is the same.

So the sentence first says what does not affect the result, and then gives the result.

That is a very common Japanese pattern:

• X にかかわらず、Y
• Regardless of X, Y

This is a useful structure to recognize quickly when reading formal Japanese.