mjhjmtu

so not really sure on how I can achieve this, I have tried to do it so far with tikznode and didn't get it to look like I wanted, granted not really sure of what I'm doing. This is the output I'm trying to get :

This is the basic code I'm using, without any annotation added to it :

documentclassarticle
usepackageamsmath,mathrsfs
usepackage[usenames, dvipsnames,svgnames]xcolor
usepackagetikz
usetikzlibraryintersections, decorations.pathreplacing,shapes,shapes.geometric,hobby,patterns,babel,matrix,calc
DeclareMathOperatorMatMat
begindocument
$L_ij(k) rightarrow E_ij(k) = beginpmatrix
1 & & & \
& 1 & & \
k & & ddots & \
& & & 1
endpmatrix$
enddocument

According to the chat, a [the new version of tikzmark] will be in the TeX distributions](https://ctan.org/pkg/tikzmark?lang=en) very soon part of the standard TeX distribution. The new version contains a command tikzmarknode, which can be used as follows:

documentclassarticle
usepackageamsmath,mathrsfs
usepackage[usenames, dvipsnames,svgnames]xcolor
usepackagetikz
usetikzlibrarytikzmark,calc
DeclareMathOperatorMatMat
begindocument
$L_ij(k) rightarrow E_ij(k) = beginpmatrix
tikzmarknodel11 & & & \
& tikzmarknodel21 & & \
tikzmarknodekk & & ddots & \
& & & tikzmarknodel31
endpmatrix$
begintikzpicture[overlay,remember picture]
draw[<-] ($(l1.center)!0.5!(l2.center)$) to[out=45,in=180] 
($(l1)+(2.5cm,0)$) node[right,anchor=north west,align=left,yshift=baselineskip]some long text\
and more text;
draw[<-,shorten <=3pt] (k.west) --++ (-0.4,0) node[left] text;
draw[<-,shorten <=3pt] (k.south) --++ (0,-0.8) node[below] more text;
endtikzpicture
enddocument

Now you need still to download the updated library for this to work, but in a bit you wont' have to worry any more.