A lista foi feita pela minha mãe, então foi fácil comprar tudo.

Foi feita is the passive voice in the pretérito perfeito (simple past).

• foi = past of ser (to be), used to form the passive
• feita = past participle of fazer (feito/feita)
  So A lista foi feita... = The list was made... (completed action in the past).

Because the participle agrees with the noun it describes. Lista is feminine singular, so the participle must be feita.

• o relatório foi feito (masc. singular)
• a lista foi feita (fem. singular)
• as listas foram feitas (fem. plural)

Pela is por + a (a contraction). Since mãe is preceded by the definite article a (here included inside minha in the sense of a minha mãe), Portuguese contracts it:

• por + a = pela
  Similarly: por + o = pelo, por + os = pelos, por + as = pelas.
  In passive sentences, por/pelo/pela commonly introduces the “agent” (who did it).

It’s not necessarily “wrong,” but it changes the feel.

• pela minha mãe sounds more natural/typical because Portuguese often uses the article with family members in this kind of structure.
• por minha mãe can sound a bit more formal, emphatic, or like you’re deliberately omitting the article.

Both exist. In Brazil, you’ll often hear minha mãe without the article in everyday speech. But when it follows por, the contracted form pela minha mãe is extremely common and natural (because pela already contains a).

Here então means so / therefore (showing a result).
It often appears in the middle like this:

• ..., então foi fácil...
  You can also place it at the start of the second clause for emphasis:
• ..., então, foi fácil comprar tudo.
  Or:
• A lista foi feita pela minha mãe. Então foi fácil comprar tudo.

Because there are two separate past statements:
1) A lista foi feita... (event: the list got made)
2) foi fácil comprar tudo (evaluation: it was easy)
Portuguese doesn’t avoid repetition of foi the way English sometimes might; it’s normal.

The structure é/foi + adjective + infinitive is impersonal:

• Foi fácil comprar tudo. = It was easy to buy everything.
  There isn’t a normal noun subject, so the verb stays singular (foi). You don’t make it plural because nothing plural is “the subject” grammatically; comprar tudo is an infinitive clause functioning like an idea.

Not in this meaning. tudo means everything (a pronoun referring to “all the things/items”).
todos usually means everyone (people) or all (plural) but it normally needs a noun or a clear plural reference:

• comprar tudo = buy everything (all items)
• comprar todos os itens = buy all the items
• comprar todos would sound like “buy them all,” and without context it’s odd (and could even suggest people).

Portuguese often uses easy/difficult + infinitive without para:

• É difícil entender isso.
• Foi fácil comprar tudo.
  You can use de in some cases (especially with certain adjectives), but fácil + infinitive without a preposition is very common and natural.

Yes: passive vs active focus.

• Minha mãe fez a lista (active) focuses on who did it.
• A lista foi feita pela minha mãe (passive) focuses on the list (the thing affected), and the doer is added with pela minha mãe.
  Both are correct; the choice depends on what you want to emphasize.

• foi feita (ser + participle) describes the action/event: the list was made (someone made it).
• estava feita (estar + participle) describes the resulting state: the list was already done/ready.
  Example: Quando cheguei, a lista já estava feita. = When I arrived, the list was already done.

Normally mãe is lowercase: minha mãe.
You might capitalize Mãe in direct address or as a title in some contexts, but in a normal sentence describing your mother, lowercase is standard.

Yes, por/pelo/pela can also mean things like through, around, for, because of, depending on context. But in a passive sentence (foi feita), pela minha mãe is straightforwardly the agent: by my mother.

Because the sentence has two independent clauses:

• A lista foi feita pela minha mãe
• foi fácil comprar tudo
  The comma helps mark the pause and shows that então is linking the cause and result. It’s good style, especially in writing.