Learn Hardcore Vietnamese: Bác sĩ đó giỏi vì ông ấy luôn giúp mọi người. - That doctor is good because he always helps people.

đó is a demonstrative meaning “that,” used for something farther from the speaker or already mentioned in context.
này means “this,” used for something close to the speaker or just introduced.
Since we’re talking about a doctor mentioned or pointed to earlier (or not right next to the speaker), bác sĩ đó is the natural choice.

What exactly does giỏi mean here, and how is it different from tốt?

giỏi describes skill, competence, or proficiency (e.g., giỏi toán = good at math).
tốt describes general goodness, quality, or moral virtue (e.g., người tốt = a good person).
In this sentence, giỏi highlights that the doctor is skilled or excellent at what he does.

Why is vì used to introduce the reason? Could we use bởi vì, and is there any difference?

vì alone is a concise conjunction meaning “because.”
bởi vì is slightly more emphatic or formal but means the same thing; you can choose either.
Often in spoken Vietnamese, vì is enough. If you want extra emphasis or formality, you can say bởi vì ông ấy luôn giúp mọi người.

Why do we say ông ấy for “he”? Could we use anh ấy, or leave the pronoun out altogether?

Vietnamese pronouns reflect age, status, and formality.
• ông ấy refers to an older man or is a respectful form for a male.
• anh ấy is for a younger or same-age male, more casual.
You can omit the pronoun (lược chủ ngữ) if context is clear:
• Bác sĩ đó giỏi vì luôn giúp mọi người.
But including ông ấy makes it explicit who is helping.

Why is the adverb luôn placed before the verb giúp instead of after it?

In Vietnamese, most adverbs of frequency (luôn, thường, hay, seldom, etc.) precede the verb they modify.
So luôn giúp literally means “always helps.” Putting luôn after the verb would sound unnatural.

What does mọi người mean? Is it different from tất cả mọi người?

mọi người means “everyone” or “everybody.”
tất cả mọi người is more emphatic: “all people” or “every single person.”
In everyday speech, mọi người is sufficient. tất cả just adds extra emphasis.

Could we say giúp đỡ mọi người or giúp cho mọi người instead of giúp mọi người?

giúp by itself already means “help.”
giúp đỡ is a bit more formal or literary, and perfectly correct.
giúp cho adds the preposition cho (beneficiary marker). It’s also correct but slightly wordier.
Native speakers often prefer the concise giúp mọi người in everyday conversation.

Can we move the vì clause to the front, as in “Vì ông ấy luôn giúp mọi người, bác sĩ đó giỏi.”?

Yes, you can front the reason clause for stylistic variation:
“Vì ông ấy luôn giúp mọi người, bác sĩ đó giỏi.”
When you do, use a comma after the vì clause.
Putting the cause first shifts the emphasis onto why he is good, whereas the original emphasizes that he is good before explaining why.