One future tense for every verb: what Serbian built that Russian never needed to
Neither Serbian nor Russian kept a single inherited future-tense form from Common Slavic - both had to solve the same missing piece some other way. Serbian's solution is one auxiliary that works on every verb regardless of aspect. Russian's solution is not a solution at all in the same sense: it split the job in two and let the aspect system itself do most of the work. A Russian speaker's instinct to let aspect decide the construction is exactly right at home and exactly wrong in Serbian.
Common Slavic did not hand its descendants a working future tense the way it handed them a past. Every modern Slavic language had to build or repurpose something to fill that gap, and Serbian and Russian took two genuinely different routes - not a shared inheritance with minor spelling differences, but two separate repairs to the same missing piece of grammar.
Serbian's fix: one auxiliary, blind to aspect
Futur I is built from the short present-tense forms of хтети, namely ћу, ћеш, ће and so on, attached to an infinitive, fused or not, as an earlier article on this blog covers in full - and it applies to every Serbian verb equally. It does not ask whether the verb is imperfective or perfective first. Both simply plug into the same construction.
Радићу цео дан.
I'll be working all day.
Imperfective радити in Futur I - the ordinary, drawn-out sense of the future.
Урадићу то до петка.
I'll get it done by Friday.
Perfective урадити, the exact same construction - same auxiliary, same fusion rule, nothing about it changes for the other aspect.
Russian's fix: let the aspect system carry the weight instead
Russian never grammaticalised хотеть into a future auxiliary the way Serbian did with хтети. Instead, the job fell to aspect itself, and aspect answers it in two completely different ways depending on which one the verb belongs to. A perfective verb has no dedicated future form at all: its ordinary present-tense conjugation, сделаю for instance, is automatically read as future, precisely because a perfective verb cannot describe an action actually in progress. An imperfective verb has no such shortcut, so it needs a real two-word construction instead - буду plus an imperfective infinitive, as in буду делать.
Where the transferred instinct misfires
A Russian speaker learning Serbian brings a perfectly reasonable habit: check the aspect first, then decide which construction the sentence needs. That habit is exactly right in Russian and exactly the wrong question to be asking about a plain Serbian future statement, because Serbian already answered it - the aspect only decides which infinitive goes into Futur I, never whether Futur I applies at all.
There is no Serbian equivalent of reaching for a two-word periphrastic construction only for one aspect and a bare conjugated form for the other. Both радити and урадити go through ћу/ћеш/ће…; a learner searching for a Serbian counterpart to буду делать specifically for imperfective verbs, as if the plain present of a perfective verb already covered the perfective side on its own, is solving a problem Serbian does not have.