Or more accurately, the Taiwanese government also believes that mainland China and Taiwan should be unified (ie. a One-China Interpretation). But that this One-China should be under the rule of the Taiwanese government and not the CCP, which they considered an illegitimate government, up until the 1992 Consensus.
After the 1992 Consensus [1], the Taiwanese government still considered the Mainland its territory (again under a One-China Interpretation), but also acknowledges the CCP's interpretation of One-China. In practice, this meant they officially abandoned plans to re-take the Mainland, and focus on maintaining the status quo of peace and stability.
Interestingly, the Taiwanese government also used to lay claim to Mongolia in addition to the Chinese Mainland.[2]
[1]https://en.wikipedia.org/wiki/1992_Consensus
[2]https://www.taipeitimes.com/News/feat/archives/2024/08/25/20...