Question: Why is the seawater saline and not fresh?

Hi!
This is a really interesting question!
First of all, all the water (also rainwater and river water) contains a mix of chemicals dissolved in it which are generally called “salts”.
When rainwater flows over rocks and through the soil it dissolves some of the minerals and incorporates them in its composition (this process is called “weathering”). This water will then reach rivers and get transported to the ocean.
When the Sun’s heat causes this water to evaporate from the ocean, it leaves the salts behind. This evaporated water will then fall again as rainwater, collect more salts through weathering, and eventually reach the ocean again, as part of the “hydrologic cycle”. It took millions of years and continuous iterations of the hydrologic cycle to build up the current content of salt in the ocean, which made seawater taste “salty”.

There are also other processes which bring salts to the ocean, such as volcanoes erupting underwater and “hydrothermal vents”, which are areas on the ocean floor where the seawater has become hotter after flowing through rocks of the oceanic crust. This water dissolves some of the minerals from the crust which are then added into the ocean.

Some events in the Earth’s history have also subtracted salt from the ocean. An example is the “Messinian Salinity Crisis”, which happened between 5 and 6 millions of years ago, and during which about 6% of the world’s salt content was deposited in the Mediterranean Sea. This is why these events are called “salt giants”. Nowadays there are still about 1.5km of salt lying beneath the Mediterranean Sea, so effectively global salinity was lowered as a consequence of that event and that salt is still locked away.

It is also important to note that salinity isn’t the same everywhere. For example it’s lower close to the poles where it is diluted as a consequence of ice melt, or higher in marginal basins like the Mediterranean Sea where evaporation rates are higher than the input from rainfall and river runoff.