The evolutionary pathways that produced the flowering plants (the Angiosperms) have been studied for at least 140 years.

The flowering plants appear very suddenly (geologically speaking) in the fossil record, at around 100 million years ago. Before that, all known plants were Gymnosperms, which had no flowers as such, and did not rely on pollinating insects (etc.)

Charles Darwin characterised the sudden appearance like this :

“The rapid development as far as we can judge of all the higher plants within recent geological times is an abominable mystery.”

A 2021 paper in the American Journal of Botany describes the history of Darwin's investigations.

BBC News interviewed the author. Professor Richard Buggs of Queen Mary, University of London, in 2018 "One hundred and forty years later, the mystery's still unsolved,"

To summarise, although recent progress in fossil examinations has partly clarified the evolution (the current consensus is that the flowering and non-flowering plants had a common ancestor) much of the detail is still unknown.