The division of time into discrete historical periods is always somewhat arbitrary, although it is helpful in discussing history. Historical transitions develop over time, and there is no precise year when the Medieval ends and the Modern begins. A scholar may note a particular event as a crucial turning point, as your sources seem to have done, but there is no "right" answer, and most scholars would give a range or approximate time. Sometimes they may disagree entirely on how we should look at time, especially if they are working in different geographical areas or on different topics.

The two dates you mention are certainly significant in the shift from a Medieval to Modern culture, but there are other cultural changes occurring before and after. Your sources seem to be focusing on trade and exploration as the key factor, since 1453 was the fall of Constantinople which impacted trade with the East and 1492 was Columbus arriving in the Americas.

Scholars have argued for centuries now as to what exactly sparked off the renaissance but its probably more useful to think of it as a gradual crystallisation of societal, cultural and scientific changes that all pointed to a new era in human thinking. I'm guessing the 2 dates you give correspond to the fall of Constantinople and either Columbus setting off on his first voyage or the end of Muslim resistance in Iberia. So these are all important events that locally set the stage for the period we retrospectively call the renaissance but were independent events that cannot be said to have caused each other. The fate of Constantinople did have particularly wide reaching consequences though and one school of though you might want to look into on this topic is the importance of Venice in sparking the renaissance. Firstly because if was the main hub for trade with the Islamic world and many translation of important Muslim science were made in Venice, the most important at the time probably being the Canon, a medical codex thought to be ahead of european medicine. Secondly, Greek scholars had been flowing from Constantinople to Venice since its sack during the fourth crusade until its fall to the Ottomans and many have suggested that it was these migrations that sparked the renewed study and translation of both Greco-Roman. Its also worth noting the role of improving printing technology around this time making information much more widely available and accessible. The first ever printed Quran apparently came out of Venice in 1537. You could point to individual paintings or sculptures or poems but I dont think you will ever find one event or year that can really be said to have started the whole thing off. Improvement in technologies, quality of life, outlook on the world, humanist thought, movement of people to cities and the beginnings of a middle class all just made for an exciting time of development for the world and it was only we had been through it that scholars realised what a formative time it was the world they now lived in