How to get razor sharp focus + portrait editing ideas?

[KmH]

and especially with D3400 which has only 11 focus points.

The D3400 is a consumer grade, entry-level DSLR and uses Nikon's most basic Multi-CAM 1000 AF module. All of the D3x00 series use it.
Regardless how many focus points it or other cameras have they can only use 1 focus point to set the focus.
The camera(s) have options that let the camera choose 1 of the focus points available rather than the photographer choosing one.

Unfortunately, Nikon elected to not put the Focus section (and other sections) in the D3400 Reference Manual.
Download the D3300 Reference Manual because it uses the same Multi-CAM 1000 AF module your camera has but has the Focus section that explains the options it has.

There are 2 kinds of focus points - cross-type is the more accurate type and can detect both horizontal contrast and vertical contrast, and - single-type, that depending on how they are oriented, detect vertical or horizontal contrast.
The Multi-CAM 1000 AF module has just 1 cross-type focus point. It's the middle one.

Understanding Camera Autofocus

 

[KmH]

But if the ISO speed is manually set (as in this case at 200) then the exposure is not affected by exposure compensation. Rather, the metering will now read 1/3 stop darker just to help you make exposure decisions; it doesn't actually change the exposure.

I did not know this.... thank you!

Using a Nikon in M - manual mode, when you adjust EC it just biases the light meter indicating the compensation amount you chose. You have to change the exposure setting(s) then to make put the light meter scale back on zero.

The bottom line is that the better you understand how the camera works from a technical perspective, the better you will be at controlling it so it does what you want it to do within it's limitations.

 

[Inklingforsake]

and especially with D3400 which has only 11 focus points.

There are 2 kinds of focus points - cross-type is the more accurate type and can detect both horizontal contrast and vertical contrast, and - single-type, that depending on how they are oriented, detect vertical or horizontal contrast.
The Multi-CAM 1000 AF module has just 1 cross-type focus point. It's the middle one.

Understanding Camera Autofocus

I just went through this article you shared. I understand better now. Thank you so much!