No company reached a $1 trillion market capitalization in the 20th century; Apple did so first in 2018.
The first widely recognized public-company milestone at $1 trillion came when Apple crossed the threshold in August 2018. That date was more than 18 years after the 20th century ended, so the premise has no company answer for that period.
Market capitalization milestones became more common as stock markets expanded, inflation increased nominal company values, and large technology businesses achieved global scale. Before Apple, companies such as Microsoft, General Electric, and ExxonMobil were among the world's most valuable, but none reached a trillion-dollar market cap during the 1900s.
This fact also illustrates why market-cap comparisons across decades require caution. A dollar in 1900, 1950, or 2000 did not have the same purchasing power, and share counts can change through splits, buybacks, and new issuance. The milestone refers to nominal market value at the time.