I picked up 2k20 when it was on sale last year and played quite a bit through lockdown, so I'm fairly new to the series, but I definitely went through a similar steep learning curve.
In terms of defending, I think it's best to keep it simple. I presume you have the indicator switched on which shows you which player to cover? If so, use a combination of L2 and R2 to stay close to that player, but always try to stay 'goal side' of them. You can also hold the right stick up whilst doing that, which will make your defender raise his hands. The combination of L2, R2 and right stick is often sufficient to slow the attacking team down and force them into taking a difficult shot.
Once you're comfortable with that, you can also use triangle to jump/block when the attacker gets near the basket. However, try not to get frustrated when you concede even after doing everything correctly because good players will often score anyway - plus, you only need to prevent a few of the opposition's attacks to gain the lead in a match.
Also, I almost never attempt to steal the ball because it results in a foul most of the time. However, occasionally the L2/R2 combination puts the attacker under enough pressure so that they drop the ball and you can grab it then.