Are you developing a Product or participating in a Project? Project is the tactic for product strategy. So you better learn about how that project affects the product(s) it relates to.
Just like in real life a Product is an always evolving result of some organized work.
A product has a lifecycle and as in life it will begin and someday disappear to give its space to something better. Nothing lasts forever.
The Product gets out in the market, it is then maintained and enhanced, until its end of life (EOL) comes.
In all product phases there are components to build, fix and maintain. Even to retire the Product from the market you might need to develop some components. These product components can be fixes to existing defects, simple cosmetic changes, features that will take a unit o work (for example 8 hours worth of overall work), several units of work or multiple units of work, etc.
If the units of work are to be calculated for absolutely all features you will be probably generating too much waste. Lean thinking teaches us to triage, so save your time on estimations when you can. In those cases consider yourself just being "maintaining your product".
There are cases in which the units of work must be calculated, for example a fixed delivery date feature is needed and it becomes clear the effort will go above certain amount of time and there is absolutely no way to push just small increments to production (It is an all or nothing situation). For these or any other (questionable perhaps) cases where it is decided to define a Project there will be a need for a meeting (estimation) involving the needed subject experts to decide what features will be grouped together (scope), how many *dedicated* people and materials will be needed (resources) and an agreement on a timeframe for completion (schedule). At that point and only then we should consider a "Project" has been born.