I want to create the nsis installer like Notion's (desktop version on windows). I already know that Notion installer is created via nsis. It is not the default installer from nsis but one that is made with a specific script. Since it's not the first time I've seen an installer like Notion's, so I assume that this script for NSIS is not a private one.
When you run the installer, it installs immediately, instantly. The installation window is small (rectangular). The name of the window is "Notion Setup". And in the window itself, there is a green progress bar that shows the installation process. Above the progress bar is the text "Installing, please wait...".
Also what I can add is that if you open the installer as an SFX archive, you can see two folders in it. The first folder is called $PLUGINSDIR and the second folder is called $R0. In the $PLUGINSDIR folder there are the following files: app-64.7z, nsExec.dll, nsis7z.dll, SpiderBanner.dll, StdUtils.dll, System.dll, WinShell.dll. And in the folder $R0 there is one file Uninstall Notion.exe.
NSIS does not offer out of box the solution for such specific design.
But its definitely possible to create such setup in script, of course you need to tweak NSIS a lot and pasting such huge scripts is beyond the StackOverflow.
What did you try? Please check e.g. nsis custom page sizes (this feature you are asking for is called custom UI)
Also you may check the Graphical Installer (it is 3rd party extension for NSIS: https://nsis.sourceforge.io/Graphical_Installer) that can make a lot of things easier for you.
P.S. I am developer of it, sorry for self promo
For start see NSIS forums, there are several good topics about that: https://forums.winamp.com/forum/developer-center/nsis-discussion/