You partially answered your own question; sanctions. Due to sanctions Iran was unable to procure much of its needed tech abroad, and thus developed a large domestic defense industry.

The other answer is their geopolitical situation. They are a highly aggressive regime, engaging in rivalries with multiple powers throughout the region, including Iraqi proxy forces against America, Hezbollah against Israel, and the Houthis against Saudi Arabia. Consequently, they are not exactly surrounded by friendly nations, so they can't just sit on their hands, they need to maintain and upgrade their capabilities to stay afloat in this shitty mess they created.

So that explains why,as for how, well regardless of their geopolitical situation and ass-backwards government, they are still a large country, with a large and highly educated citizenry to pull from from to develop such capabilities. To put it into perspective, their population is ~86 million, more than half of Russia, about twice that of Ukraine, and more than either France or the UK. Combine that with a territory larger than either France or the UK, some of the world's largest petrochemical reserves, plus plenty of available field-testing via the (mis)adventures of its proxies, and you have many of the makings of a military competent regional power with an indigenous defense industry to match.

Now, you may ask why Russia, which fits many of the same criteria (belligerence,size, sanctions, petrochemical wealth) didn't advance at the quite the same speed? The answer is in two parts, first off they did, just not in large numbers, their new hyper-sonic missiles (khinzal), new tanks (Armata), and new suicide drones (lancet) actually are combat effective, but it does not matter because they don't exist in large enough quantities to affect the war. It doesn't matter if you engineered the new Armata tank if you are still using t-60s. This ties in to the second part, why didnt they build enough? I think its because of one fundamental difference, Iran sees itself in an existential conflict, its very existance, or rather that of its regime, is under threat, to them military progress in effective numbers is a must. As for Russia, barring the tumultuous 90s, Russia sees itself as a great power, if not super-power, one of the storied, established ones, far more akin to France than Iran. Thus, lacking the same existential dread, and buoyed by history and arrogance, they allowed themselves to pursue military advancement more as a vanity project, or to threaten the west, nothing more. It simply wasn't necessary. Until fighting a Westernized Ukrainian army showed them otherwise.

>never being a global power

P.S. , just to clarify, Iran was one of the worlds original empires, under the Achemenids, Parthians, Sassanids, etc.