Jo no acostumo a portar botes a casa, però a la muntanya sí.

Not strictly. Catalan often drops subject pronouns because the verb ending already shows who is doing the action.

• acostumo already means I usually / I tend to
• So No acostumo a portar... is perfectly natural

Including Jo adds a bit of emphasis or contrast, something like:

• As for me, I don’t usually wear boots at home...

So Jo is possible, but not required.

Here acostumo a + infinitive means I usually..., I tend to..., or I’m in the habit of...

So:

• acostumo a portar = I usually wear

This is a habitual meaning. It does not mean I am used to in the English sense of being accustomed to something.

Because this pattern is:

• acostumar a + infinitive

So you say:

• acostumo a portar
• acostumes a menjar
• acostumen a sortir

That a is part of the normal structure of the verb in this use, so it should not be omitted.

Yes, portar can mean to carry or to bring, but it also very commonly means to wear when talking about clothes, shoes, glasses, jewelry, and similar things.

So:

• portar botes = to wear boots
• portar ulleres = to wear glasses
• portar una jaqueta = to wear a jacket

This is very normal Catalan.

Because footwear is usually referred to in the plural, just like in English:

• boots
• shoes
• sandals

So portar botes means to wear boots in a general sense. It does not suggest several pairs; it just refers to that type of footwear.

A casa usually means at home or home, depending on context.

In this sentence, it means:

• at home

This is a very common fixed expression in Catalan.

Examples:

• Sóc a casa = I’m at home
• Em quedo a casa = I’m staying home
• No menjo això a casa = I don’t eat that at home

Because a casa is a fixed idiomatic expression. Catalan normally says:

• a casa = at home
• not a la casa in this meaning

But muntanya is an ordinary noun here, so it takes the article:

• a la muntanya

So the contrast is:

• a casa = fixed expression
• a la muntanya = preposition + article + noun

In this sentence, it means something like in the mountains or when I’m in the mountains.

Catalan often uses a with places where English might use in, at, or to, depending on context. So you should not translate the preposition too literally every time.

Here, the idea is location or setting, not movement.

That sí means yes, I do and replaces the repeated part of the sentence.

So:

• Jo no acostumo a portar botes a casa, però a la muntanya sí

really means:

• I don’t usually wear boots at home, but in the mountains I do

The second clause omits the repeated verb phrase because it is understood.

This sí is especially common after a negative statement followed by a contrast:

• No menjo carn, però peix sí
• No hi va entre setmana, però els diumenges sí

Also, the accent matters:

• sí = yes
• si = if

Because Catalan often puts the contrasting element first.

Here the contrast is between:

• a casa
• a la muntanya

So a la muntanya comes first for emphasis, and sí confirms the opposite of the earlier negative idea.

It feels natural in Catalan to structure it this way:

• not at home, but in the mountains yes

In more expanded form, you could also say:

• ...però a la muntanya sí que porto botes
• ...però a la muntanya sí que n’acostumo a portar

But the short version in your sentence is very natural.

No porto botes a casa means:

• I don’t wear boots at home

This can sound like a straightforward fact.

No acostumo a portar botes a casa means:

• I don’t usually wear boots at home
• I’m not in the habit of wearing boots at home

So acostumo a adds a habitual or general-pattern meaning.

Yes. Another common way to express habit is soler.

So you could also say:

• No solc portar botes a casa, però a la muntanya sí

That also means I don’t usually wear boots at home, but in the mountains I do.

Both are correct, though acostumar a + infinitive and soler + infinitive may vary a bit by style, region, and speaker preference.