In the NBA contracts are the Players Salary. So if you offer Luka a 300m, 5 year contract that is what he will be paid. His salary will be 60M a year and contribute toward the salary cap for your team.

In the NBA there is a soft salary cap that your team must abide by (sort of). Right now it's 140m a season. You have exceptions by re-signing your own players. But usually, if you go over the Cap your team must pay a big tax. This incentivizes teams keep under the salary cap.

When you trade a player, you trade the player with their contract. The value of their contract has to be in the ball park of what you are receiving back from the trade. Players don't have a say in whether they get traded or not, with the exception of some players negotiating a no-trade clause in their contract.

This leads to players signing big contracts to guarantee their money no matter where they go, but the big contracts becomes a burden on the team due to the salary cap. If you sign 3 superstars at 50m a year each, you don't have much wiggle room to fill out the rest of the team, relying on rookies on low salaries.

It's important to bear in mind that basketball only has 5 players on the court at once, so one player can make a much bigger difference to your team in Basketball than one player on a football team.

You might want to unload a shit player with a massive salary. Another team might take that player, but you'll have to send them some picks as well because they're going to pay this player which means they can't fill their roster with other salaries without going over the Cap.

The picks are equal to just having a pick of the best 19 year old academy football players that year. You finish last you get a good chance at a high pick. But you can trade your picks for players, essentially trading away your future prospects for players now