Israel unilaterally declared independence following what was in essence a civil war in Mandatory Palestine. The UN had partitioned the territory but Israel took much more than had been allotted to them. Israel also illegally expelled many Palestinians and refuses them the right of return to this day, which they do counter to international law. In 1967 Israel expanded even further and took the remaining Palestinian territories which they occupy to this day in the world’s longest occupation. To this day, Israel keeps taking more land from Palestinians e.g. by setting up illegal “security corridors” or opening new illegal settlements.
So to answer the question. The land was taken from the Palestinians. And it keeps being taken from the Palestinians, in defiance of a number of UN resolutions, to whom the British had given the mandate to.
In an alternative universe where Israel wouldn’t be colonial, there would not have been a civil war, Israel would not have unilaterally declared independence, but done so in agreement with Palestinians, the UK and the UN. They wouldn’t have expelled any Palestinians, and they wouldn’t have maintained a policy of maintaining an ethnic majority. Jews would live now as a minority in Palestine, hopefully with some minority protections mandated by the UN (and probably demanded by the UK as part of the independence agreement).
In a slightly less alternative universe where the Zionist national project still happens and Israel unilaterally declares independence, at any time after 1948, in an effort to right previous colonial wrongs, Israel would offer the expelled Palestinians the right to return and reparations for their years or decades in exile. They would dismantle their ethnodemographic policies, and either integrate the occupied territories into a single democratic (non-apartheid) Israel-Palestine or recognize an Independent Palestine at the 1967 borders with some freedom of movement between the two states (similar to Ireland and Northern Ireland). For as long as non of this happens. Israel’s current policies are an unbroken link to their colonial past.