Bl Pope Pius IX made this statement ("I am the Church") in regards to his power to define the dogma of the Immaculate Conception, which he did define in 1854. A few years prior to this statement and dogma, he was forced to escape Rome because of a revolution where the Prime Minister was αssαssιnαtҽd. So, for context, 1) he said this having escaped a life-or-death situation, 2) he was aware of the infiltration in the Church, and 3) he was getting pushback from the Cardinals, some of whom were pushing the heresy of collegiality and telling him that he must wait to define the dogma when all the bishops/cardinals agreed on the definition.
Vatican I and the doctrine of infallibility had not been defined (1870), so this pushback and call for the heretical collegiality/democracy of the Church is what prompted Bl Pius IX to lash out and correct the liberal Cardinal, when he shouted "I am the Church". The point/context of the statement is that in matters of dogma/infallibility, the pope IS ALONE.
To be clear, Bl Pius IX was not saying the pope is all that matters, or the pope can do whatever he wants in the Church, or that he has no rules to follow or that he can change doctrine or the sacraments. These matters are beyond his authority. He was simply correcting liberal Cardinals on the matter/power of the Pope in relation to infallibility/doctrine, which had not yet been clarified at Vatican I.