Learn Russian: Кто-нибудь мог подбросить бриллиант в карман? - Could someone have planted the diamond in the pocket?

In Russian, the most important or 'new' information (the focus) goes at the end of the sentence. By ending with в карман, the speaker focuses on the location: the pocket. Because бриллиант comes before the location, it is treated as known information (translating as 'the diamond'). If the order were в карман бриллиант, the diamond would become the new, indefinite focus, translating closer to 'Could someone have planted a diamond in the pocket?'

Why use кто-нибудь here instead of кто-то?

Кто-нибудь is used for a completely hypothetical or undefined person ('anyone at all'), which is the standard choice for speculative questions like this. Кто-то refers to a specific, existing person whose identity is just unknown to the speaker ('someone specific'). Since the speaker is asking a theoretical question about whether this action was even possible, кто-нибудь is the natural choice.

Doesn't подбросить mean to throw something up in the air or to give someone a ride?

It does, but it has a third very common meaning. The prefix под- often implies doing something secretly or underhandedly. In contexts involving crime, evidence, or tricks, подбросить means to secretly plant something on someone or in their belongings.

The English translation says 'Could someone have planted', but подбросить is just an infinitive. How does Russian show the past tense here?