@Jeyganesh There is no easy way of merging tasks from multiple task lists into one view.
You can (if You have the Modern Experience) create a page and, on the Site Collection root site, insert the web part "Highlighted content" and configure it to only show Tasks. Perhaps not the best visual experience but at least you will get a list of all of your tasks.
If You are using the Classic Experience You can place a Search Result Web part on a page and configure the query so that it only displays tasks from the current site collection.
Otherwise i would consider using "Planner" instead of SharePoint task lists. That givs You a much better overview and experience.