Recently, I have found out about 100’000 to 200’000 Zoroastrians still exist. But I was wondering how? The Islamic conversions of Iran definitely wouldn’t have let them stay right? How did they avoid religious extinction for so long?

Keep in mind that even after long periods of time (hundreds of years in some cases) Islam was often not the majority religion in areas conquered by Muslims, and to a degree this does apply to Iran. Zoroastrians likely became a minority during Abbasid rule at least a hundred years after the initial conquest. Policy varied significantly based on time, dynasty, and ruler. While persecution of non-Muslims was normal it was (esspecially initially) not a "let them exist" type situation until later. There were incentives to convert but many groups were tolerated to varying degrees including Zoroastrians. Fire temples existed for hundreds of years in Iran even after the Muslim conquests.

Military expansion is obviously violent but once conquered the actual initial majority conversion of many of these areas was largely non-violent. That's not to discount the large scale persecution and even massacres that happened (often worse later not earlier) but the main reason Zoroastrians exist is because for much of their history they were a clearly tolerated minority under Muslim rule.

In Islam there are concepts like "Ahl al-Kitaab" (which translates to "people of the book") and the status of "ahl al-dhimma". Holy text like the Bible or Torah are recognized as divine and grant a protected status to people like Christians, Jews, and Sabians who qualify as "people of the book". Other groups can also be given some of this Dhimmi protected status and recognized as tolerated non-Muslim people under Islamic rule. There was a lot of debate as per what qualified and what that meant, Zoroastrians were often at the center of this discussed as both with varying interpretations. These are not static concepts and there was/is active scholarly debate on what grounds someone qualified and what that meant. When it came to Zoroastrians they are more generally accepted than some but there has been serious questions about the "divine revelation" of the religion compared to the Jewish/Christian peoples and this did often lead to protections wavering and changing depending on many factors. Muslim scholarly debate about Zoroastrians has a very wide range of opinions, keep in mind there is a Hadith guessing that 124,000 prophets were sent out to the people of the world, there is a lot of room for debate on what that means about other groups relation to Islam. Just as a side note the "people of the book" concept has proven practically useful, for example some Muslims (involved with conquering or ruling much of India) considered even Hinduism divinely inspired from text as a justification for relatively tolerant policy (even if many outside of those areas rejected it). Hindus did receive protected status as dhimmi and Hinduism remain a majority in India despite long periods of Muslim rule.

As a reading source I recommend the short paper "Dhimmi or not Dhimmi?: Religious Freedom and Early Islam" by historian Mark W. Graham.