How to clean up a Linux installation?

I've been using Linux (mainly Ubuntu) for about two years now. I've had several installations on several machines. Something that inevitably happens on any OS installation is that I install something I need that day, maybe a library or CLI utility, use it for a week, and then forget about it. I'm sure I've got several GB of unused packages taking up space on my installations. How can I clean up my installation to make sure that I only have things that I want? I'm aware of apt-get autoremove to remove unused dependencies. I'm aware of apt list --installed to view all packages installed through apt. I'm aware of dpkg -L to view all files installed from a package. Suppose I were to look though the output of apt list --installed and remove things I don't want by either apt-get purge or deleting every file from dpkg -L . Eventually, I would have cleaned up everything installed through my package manager, right? Where else should I look? Not everything on my computer was installed through apt. For example, .deb packages like google-chrome and things installed via scripts. Where would these things live? And what's the best way to find them? My goal is to shrink my installation down to basically a fresh install plus the things I use all the time. Thanks, all!