الكاميرا التي في هاتفي جيدة، لكن الشاشة ليست كبيرة.

التي is the relative pronoun meaning that / which for a feminine singular noun.

It refers back to الكاميرا, so it has to match that noun in gender and number.

• الذي = masculine singular
• التي = feminine singular

So:

• الكاميرا التي... = the camera that...
• not الكاميرا الذي...

Because Arabic nouns have grammatical gender, and كاميرا is treated as feminine in Modern Standard Arabic.

That is why it takes feminine agreement:

• الكاميرا
• التي
• جيدة

Even though كاميرا does not end in ة, it is still feminine by usage.

هاتف means phone.

When you add -ي, it means my:

• هاتف = phone
• هاتفي = my phone

So في هاتفي means in my phone or on my phone, depending on context.

Both are possible, but they are built differently.

1. الكاميرا التي في هاتفي

   • literally: the camera that is in my phone
   • this uses a relative clause

2. كاميرا هاتفي

   • literally: my phone’s camera
   • this uses an iḍāfa possessive structure

The version in your sentence is a little more explicit. It identifies the camera by saying it is the one in my phone.

Yes, literally في means in.

Here that makes natural sense, because a camera is physically part of a phone. In English, you might sometimes say on my phone, but in Arabic في هاتفي is completely natural for something that belongs to or is contained within the phone.

In present-tense Arabic nominal sentences, the verb to be is usually not written or spoken.

So:

• الكاميرا جيدة = the camera is good
• literally: the camera good

This is normal Arabic sentence structure.

Because it agrees with الكاميرا, which is feminine singular.

In Arabic, predicate adjectives usually agree with the noun in:

• gender
• number

So:

• masculine singular: جيد
• feminine singular: جيدة

Since الكاميرا is feminine, you need جيدة.

Because الشاشة is the subject and it is definite: the screen.

But كبيرة is the predicate adjective, and predicate adjectives in Arabic are usually indefinite.

So:

• الشاشة = the screen
• كبيرة = big

This is the normal pattern:

• البيت كبير
• السيارة سريعة
• الشاشة ليست كبيرة

Arabic normally uses ليس to negate a nominal sentence in the present.

So:

• الشاشة كبيرة = the screen is big
• الشاشة ليست كبيرة = the screen is not big

Here ليست is the feminine singular form of ليس, because الشاشة is feminine singular.

So the agreement is:

• ليس = masculine singular
• ليست = feminine singular

Because الشاشة is a feminine singular noun, and ليس agrees with the subject.

Compare:

• الكتاب ليس كبيرًا = the book is not big
• الشاشة ليست كبيرة = the screen is not big

So the ت in ليست shows feminine singular agreement.

Yes, you can think of it as having an implied هي.

So:

• التي في هاتفي can be understood as
• التي هي في هاتفي

In English, that would be that is in my phone.

Arabic often leaves out that pronoun in this kind of structure.

It is basically two nominal sentences joined by لكن.

1. الكاميرا التي في هاتفي جيدة

   • الكاميرا التي في هاتفي = subject/topic
   • جيدة = predicate

2. لكن الشاشة ليست كبيرة

   • لكن = but
   • الشاشة = subject/topic
   • ليست كبيرة = negative predicate

So the pattern is:

• statement 1
• لكن
• contrasting statement 2

لكن means but.

Yes, ولكن is also possible. Both are common in Modern Standard Arabic.

• لكن = shorter form
• ولكن = fuller form

In this sentence, either would sound natural.

Yes. A compact version would be:

كاميرا هاتفي جيدة، لكن شاشته ليست كبيرة.

This means the same basic thing, but it uses:

• كاميرا هاتفي = my phone’s camera
• شاشته = its screen / his phone’s screen

Your original sentence is still perfectly correct; it is just a little more explicit.