Is there any way to mark a file inside a folder to locate it easily and quickly? For example, with a mark on top of the file image.
Or to create a link/bookmark to that file in Dolphin that when I select it will show me that file in the folder that contains it.

You can always drag the particular file somewhere at which point the contextmenu will offer to create a link to that file. (The key combo is ctrl+shift.)https://imgur.com/a/VFuv95w
You can always create a servicemenu which would, say, link stuff to a preset destination, or..
Since the copy/move bug still hasn't been fixed ( those context entries don't work on desktop, only in Dolphin itself) I created a copy and move script using zenity.
So, a link servicemenu would look like this, popping up a zenity destination window. There are more ...plasma-ish ways to achieve it but this is easy and lightweight.
Or, you could create some folder and put it in your sidebar. Drag n link also works towards the location bar.
Edit: I'm still awake and quickly made a servicemenu. You need zenity of course. File goes in ~/.local/share/kservices5/servicemenus.
Result: https://imgur.com/a/rxiYyQX

[Desktop Entry]
Type=Service
Icon=edit-link
X-KDE-ServiceTypes=KonqPopupMenu/Plugin
MimeType=all/allfiles;inode/directory;
Actions=LinkIt;
X-KDE-Priority=TopLevel
#X-KDE-Submenu=
Encoding=UTF-8

[Desktop Action LinkIt]
Name=Link It
Icon=edit-link
Exec=ln -s %F $(zenity --file-selection --directory)

New edit.
I noticed the kdialog problem's been solved so now you can use native kdialog for a directory popup. Had some issues before.
Anywho, made this submenu for copy/move/link any kind of file. Looks like: https://imgur.com/a/dW2j8hn

Like I said, the native copy/move still doesn't work on the desktop ( only in dolphin itself), that's why I added the copy/move as well.
Goes in the same directory, ~/.local/share/kservices5/ServiceMenus. As you notice, the link action is called zinkit. That's because those servicemenus go alphabetically. I still don't know how to assign an icon to the top level entry, plasma takes the first icon of the submenu.

[Desktop Entry]
Type=Service
X-KDE-ServiceTypes=KonqPopupMenu/Plugin
MimeType=all/allfiles;inode/directory;
Actions=copyTo;moveTo;zinkIt
X-KDE-Priority=TopLevel
X-KDE-Submenu=CML actions
#X-KDE-Submenu=
Encoding=UTF-8

[Desktop Action copyTo]
Name=Copy It
Icon=edit-copy
Exec=cp -r %F $(kdialog --getexistingdirectory)


[Desktop Action moveTo]
Name=Move It
Icon=edit-move
Exec=kdemv5 %F $(kdialog --getexistingdirectory)

[Desktop Action zinkIt]
Name=Link It
Icon=edit-link
Exec=ln -s %F $(kdialog --getexistingdirectory)

Thanks, but this does not work for me. I don't want to open the file, but to locate it easily inside a folder, in the order they are displayed.

Ok. Can't help you then. If you want to colorize or something a file/folder within its directory, you're out of luck. I believe Macos and Windowz have such a feature, plasma doesn't. At most you can colorize a folder.

You mean something like this right? https://imgur.com/a/FrzH4uP

dzon wrote:You mean something like this right? https://imgur.com/a/FrzH4uP

Yes, just that.

Nope, not available in dolphin. Just tagging. Nemo, Caja and such have emblems and such, dolphin not.