(A) は (B) となっている is a common expression used to show that ‘(A) is becoming (B)’, or (more commonly) ‘(A) has become (B)’. This expression will only appear with nouns, or な-Adjectives in their 語幹（ごかん） (stem form). (A) は (B) となっている is often translated as ‘(A) happens to be (B)’, ‘(A) has been established as (B)’, or ‘it has been decided that (A) is (B)’.
Let’s have a look at some examples, and then how this structure is formed.

• 鬼滅（きめつ）の刃（やいば）は非常（ひじょう）に人気（にんき）となっている。
  It happens to be that Demon Slayer is very popular.
• 日本（にほん）ではドラッグは法律上（ほうりつじょう）、違法（いほう）となっている。
  In Japan, it has been established that drugs are illegal, by law.
• このプールでは、飛（と）び込（こ）みが禁止（きんし）となっている。
  At this pool, it happens to be that jumping in is not allowed.

Despite the different translations, (A) は (B) となっている always has the same meaning, which can be understood as follows:
(A) は - (A) is a constant unchanging thing.
(B) と - (B) is a result. (coming form the 格助詞（かくじょし） (case marking particle) と’s role in highlighting results.
なっている - (A) is existing in the state ‘having arrived at the result of (B)’.

• 今（いま）の時代（じだい）にはインターネットは必要（ひつよう）となっている。
  It is now that the internet is necessary in the present age.
• この高校（こうこう）ではアルバイトが禁止（きんし）となっている。
  It is now that working part-time at this school is prohibited.

Due to this, the literal translation may be thought of as ‘it is now that (A) is (B)’ in almost all situations.