OK, I think I understand but I dont think you read the link in my reply called Writing A Quality Post. It would be handy to at least know the Operating System on both your iPhone and Laptop.
Methods and capabilities vary from one OS and iOS to another.
Still this is how I do it and how I think Apple imagines we should do it.
When you connect your iPhone/iPad to your laptop and iTunes is opened you will see in the side menu bar a list (expand it if you dont) of Music, Pictures, Info etc.
Click on Music and you will have a window which has two options. One option is; to load only checked playlists, artists, genres etc. I have this selected.
Scroll through the options in the boxes below and select what you wish to load onto your device. If you wish to load single tracks from an album you can place singe tracks into a Playlist on the iTunes main window called eg Single Tracks. To load them onto your device just select Single Tracks from the Playlists box.
If you are using the Manually Manage Music and Movies option I assume it's to allow adding to your music from a different computer without erasing the library you already have BUT iTunes will not let you transfer purchased music (from iTunes) from another iTunes account (music library) to an unauthorised device. That would allow wholesale copyright infringement and illegal copies of everything iTunes sells.