Сейчас я несу тяжёлый пакет домой, а обычно ношу покупки в рюкзаке.

This is one of the most important things in the sentence.

Russian often uses two different imperfective verbs of motion:

• нести → to carry in one direction, right now, on a specific trip
• носить → to carry habitually, repeatedly, or in various directions

So:

• Сейчас я несу тяжёлый пакет домой = Right now I am carrying a heavy bag home
  This is one specific action in progress.

• обычно ношу покупки в рюкзаке = I usually carry groceries/shopping in a backpack
  This describes a habit or usual way of doing it.

So the contrast is:

• несу = now, one trip, one direction
• ношу = usually, repeatedly, in general

Yes. Both are present tense, first person singular:

• несу = I am carrying / I carry
• ношу = I carry / I usually carry

In Russian, the present tense can cover both:

• an action happening now
• a general or habitual action

The adverbs make the time meaning clear:

• Сейчас tells you it is happening now
• обычно tells you it is habitual

Because домой is a special adverb meaning homeward / to home / going home.

Russian often says:

• иду домой = I am going home
• еду домой = I am going home (by transport)
• несу домой = I am carrying something home

So you do not say в домой.

Compare:

• домой = to home, toward home
• дома = at home
• в доме = in the house/building

That is why домой appears by itself.

Because after в the case depends on the meaning:

• в + accusative = into, motion toward the inside
• в + prepositional = in, inside, location

Here the meaning is in a backpack, not into a backpack, so Russian uses the prepositional case:

• рюкзак → dictionary form
• в рюкзаке → in a backpack

So:

• кладу покупки в рюкзак = I put the shopping into the backpack
• ношу покупки в рюкзаке = I carry the shopping in the backpack

Because покупки here means shopping, groceries, or the things bought.

The singular покупка means a purchase or one bought item.
The plural покупки is very commonly used in the sense of:

• shopping
• groceries
• bought items

So ношу покупки в рюкзаке means something like:

• I carry my shopping in a backpack
• I usually carry groceries in a backpack

Yes, in this sentence пакет usually means a bag, often a plastic shopping bag.

Depending on context, пакет can also mean:

• packet
• package
• plastic bag

Here, because the sentence mentions carrying shopping, тяжёлый пакет most naturally means a heavy bag.

So the contrast is something like:

• right now: a heavy shopping bag
• usually: shopping in a backpack

тяжёлый means heavy.

It changes form to agree with the noun it describes.
Here пакет is:

• masculine
• singular
• accusative

But because пакет is an inanimate masculine noun, its accusative form looks the same as the nominative. So you get:

• тяжёлый пакет

A few related forms:

• тяжёлая сумка = a heavy bag (feminine)
• тяжёлое письмо = a heavy letter/item (neuter)
• тяжёлые покупки = heavy groceries/items (plural)

Here а shows contrast.

The sentence is comparing:

• what is happening now
• what happens usually

So:

• Сейчас я несу тяжёлый пакет домой, а обычно ношу покупки в рюкзаке.

This means:

• Right now I’m carrying a heavy bag home, but/whereas I usually carry my shopping in a backpack.

Why not и?

• и = and, simple addition
• а = and/but/whereas, often marking a contrast or comparison

Here the speaker is not just adding another fact; they are contrasting this situation with the usual one.

Yes, it could be omitted in many contexts, because the verb forms already show the person:

• несу = I carry / I am carrying
• ношу = I carry habitually

So Russian could say:

• Сейчас несу тяжёлый пакет домой, а обычно ношу покупки в рюкзаке.

That would still be understandable.

But я is often included:

• for clarity
• for emphasis
• because it sounds natural in many sentences, especially at the beginning

So the version with я is completely normal.

This is mainly about the special verbs of motion system.

Both нести and носить are imperfective, but they differ in directionality and type of action:

• нести = unidirectional, one concrete carrying action
• носить = multidirectional or habitual/repeated carrying

So the key contrast is not perfective vs. imperfective.
It is:

• specific action now → несу
• usual repeated action → ношу

This is a classic Russian motion-verb distinction.

The dictionary forms are:

• нести → to carry
• носить → to carry habitually / to carry around

The forms in the sentence are:

• несу = I carry / I am carrying
• ношу = I carry / I usually carry

These are first person singular forms.

A few more forms for comparison:

From нести:

• несёшь = you carry
• несёт = he/she carries
• несём = we carry

From носить:

• носишь = you carry habitually
• носит = he/she carries habitually
• носим = we carry habitually

Grammatically, yes, but in natural English you would usually translate it more smoothly:

• Right now I’m carrying a heavy bag home, but I usually carry my shopping in a backpack.

Why?

Because English normally uses:

• am carrying for an action happening right now
• usually carry for a habitual action

Russian present tense does both jobs, and the time words сейчас and обычно tell you which meaning is intended.