Learn Hardcore Portuguese (Brazil): Eu fiz uma lista no celular para comprar tudo no supermercado. - I made a list on my phone to buy everything at the supermarket.

Eu fazia (pretérito imperfeito) suggests a habit or ongoing background action: I used to make / I was making a list (when something happened).
Eu tenho feito (present perfect in Portuguese) usually means a repeated action up to now: I’ve been making lists (lately / recently). It’s not the normal choice for a single completed list.

What does uma lista mean here—does it sound like “a list” (one-time) or “my list”?

Literally it’s a list. Portuguese often uses uma even when English might use my if ownership is obvious from context. If you want to explicitly say “my list,” you can say uma lista (still fine) or more explicitly a minha lista.

Is no celular the same as no meu celular?

Often, yes in everyday speech. No celular = on the phone / on my phone (context usually implies it’s yours).
If you want to be explicit, especially if it matters whose phone: no meu celular (on my phone), no celular dele/dela (on his/her phone).

Why is it no celular and not em o celular?

No is a contraction: em + o = no.
Similarly:

em + a = na (na casa)
em + os = nos (nos Estados Unidos)
em + as = nas (nas lojas)

Does no celular mean “in the phone” or “on the phone”?

Portuguese uses em for locations more broadly than English. no celular commonly means on the phone (in an app/notes), not physically inside. It’s the natural phrasing for where you wrote the list.

Why is it para comprar? Is that “in order to buy”?

Yes. para + infinitive expresses purpose: I made a list on my phone *to buy everything at the supermarket.
It can also be read as “a list *for buying everything,” but the main idea is purpose.

Could I say pra comprar instead of para comprar?

Yes. pra is the very common spoken contraction of para in Brazil.

Writing/formal: para comprar
Conversational: pra comprar

Why use tudo and not todos?

Tudo is a pronoun meaning everything (a singular “collective” idea).
Todos usually means everyone or all (people/items) + a noun (or in some contexts “all of them” with agreement). For items you’re buying, tudo is the natural “everything.”

Could it be comprar tudo no supermercado vs comprar tudo no mercado—what’s the difference?

Supermercado is specifically supermarket.
Mercado can mean market (a general store, a grocery market, or even the broader idea of “the market” economically). In Brazil, people often say mercado to mean the grocery store they go to, but supermercado is clearer and more specific.

Does no supermercado mean “at the supermarket” or “in the supermarket”?

It can translate as either depending on context, but here it means at the supermarket (the place where the buying will happen). Portuguese uses em/no where English often uses “at.”

Could I also say do supermercado instead of no supermercado?

That changes the meaning.

no supermercado = at/in the supermarket (location where you will buy)
do supermercado = de + o = from the supermarket (origin/source), e.g., comprei isso do supermercado = I bought this from the supermarket.

Where can I place para comprar tudo no supermercado? Is the word order flexible?

It’s fairly flexible, but some orders sound more natural:

Most natural: Eu fiz uma lista no celular para comprar tudo no supermercado.
Also natural: Eu fiz uma lista para comprar tudo no supermercado no celular. (but this can briefly sound like you will buy “on your phone”)
To avoid ambiguity, keep no celular close to fiz uma lista.

Is it okay to omit Eu and just say Fiz uma lista...?

Yes. Portuguese frequently drops the subject pronoun when it’s clear from the verb. Fiz uma lista... sounds very natural.

Can no celular imply I wrote it using my phone, not just that it’s located there?

Yes, it implies both: you made/typed the list on your phone (so it’s stored there and created using it). If you want to emphasize the tool, you could also say pelo celular in some contexts, but for a notes/shopping list, no celular is the standard.

Would a Brazilian say fiz uma lista de compras instead?

Very commonly, yes. uma lista de compras is the set phrase shopping list.
So you could say: Fiz uma lista de compras no celular.
Your sentence is still correct; it just spells out the purpose instead of using the fixed expression.

Is para comprar tudo the same as pra comprar tudo vs pra comprar tudo o que eu preciso?

para/pra comprar tudo = to buy everything (everything on your intended list)
pra comprar tudo o que eu preciso = to buy everything I need (more explicit; “need” can imply necessities rather than just planned items).