You can copy and paste it to a new text file and save as .txt
Or if you prefer to do all from terminal:
sudo apt list --installed > installed-list.txt
or
sudo apt list --installed | tee ~/installed-list.txt
will also work.
The examples above will save to your home folder, but you can modify the command to save it to whatever directory you want. For example if in ~/Documents:
sudo apt list --installed | tee ~/Documents/installed-list.txt