Learn Hardcore Italian: La fotocamera che vedi sul tavolo è quella di cui ho perso il copri-obiettivo. - The camera you see on the table is the one whose lens cap I lost.

che is the general relative pronoun for subjects or direct objects and never carries its own preposition. In che vedi sul tavolo, che = “that” and is the direct object of vedi (“you see”).
cui on its own can only appear after a preposition. Since we need to express “the lens cap of the camera,” we use di + cui = “of which.”

Why not use la cui instead of di cui?

la cui also marks possession (“whose”), but it’s more formal and normally follows the noun directly (e.g. la fotocamera, la cui copri-obiettivo…). In everyday speech—especially after a demonstrative like quella—Italians prefer the shorter di cui (or the equally correct but more formal della quale).

What does sul mean, and why isn’t it su il tavolo?

sul is the contraction of su (on) + il (the). In Italian, simple prepositions often fuse with the definite article:

a
- il = al
da
- il = dal
di
- il = del
in
- il = nel
su
- il = sul

Why is there no tu before vedi (“che vedi sul tavolo”)?

Italian normally drops subject pronouns because the verb ending already indicates the person. vedi ends in -i, so it clearly means “you see.” Adding tu would be grammatically correct but redundant unless you want special emphasis.

Why is quella used here? Could we say questa?

quella is a feminine singular demonstrative pronoun (“that one”) matching la fotocamera.
You could use questa (“this one”) if you want to signal closer proximity or a different perspective, but quella is perfectly natural to refer back to something already mentioned or slightly more distant.

What exactly is a copri-obiettivo, and why the hyphen?

copri-obiettivo literally means “lens cover,” i.e. lens cap. It’s a compound noun formed from copri (cover) + obiettivo (lens). Italian often uses a hyphen in such compounds to clarify the connection, though in less formal texts you might also see copri obiettivo as two words.

Why do we say ho perso instead of perdo?

ho perso is the passato prossimo (“I have lost”) and is used for a completed action with present relevance (the cap is still missing). The simple present perdo would imply a habitual action (“I lose it every time”), which isn’t the intended meaning.

Could we replace di cui with della quale or del quale?

Yes. della quale (“of which,” feminine) or del quale (masculine) are more formal synonyms of di cui. In your sentence you would say:
“La fotocamera che vedi sul tavolo è quella della quale ho perso il copri-obiettivo.”
But in everyday speech di cui is shorter and much more common.