How can I improve my download speed over wifi? Wired and upload speed is fine

You pretty much can't. Those number you see are massive marketing lies. For example if you would use the same method they use on wireless you would call your ethernet cable 2gbit. They add the transmit speed and and receive speeds together. Now ethernet can technically really run that fast even though most end devices can not. Wireless though is half duplex it can only transmit or receive at any one time.

There are massive number of other technical things that makes these speeds untrue.

The numbers you are seeing on your phone are actually higher than what you see many of the testing sites get....but again there are many variables involved in the testing so you can't even compare data between sites.

The first reason you not see the speeds increases is your end equipment is 1/2 the problem. Your nexus for example likely does not support many of the new features like your samsung does and drops back to using older methods.

Most end devices for example do not have 4 antenna so they can not even attempt to use the higher level data encoding. This is the same as with the MU-Mimo features. The end device must have support or the feature is not used by your router.

Maybe it is time to step back and say do I really need it to run that fast. Other than speedtest sites you will not find sites that can come anywhere close to letting you download at very high rates. They may have artificial limits on them to protect their bandwidth. If for example some site has a 10g internet connect 10 people downloading at 1gbit each would block the site to everyone else.

You also have to do the math. How much time do you really save a day running a little faster. If some huge download takes 2 minutes longer does it really matter. For those downloads it would really save more time how long does it take to connect it via wired to do that.

Pretty much ISP connection speed is a lie also. They know that in real life usage people can not really use that much bandwidth so they can get people to upgrade from 100m to 300m even though they did not really use the 100m in the first place. Many ISP cheat on speedtest they have their network rigged to favor the site and do things like allow short download to run faster than longer downloads. A couple even outright tell you they are doing it, some have a feature called turbo that allows a certain amount of traffic to run at higher rates before it slows down.