Use a soldering iron to weld separate 3D printed plastic parts or as a cheap replacement for a hot air gun for shrinking heat shrink tubing? Nah! A soldering iron is generally too hot for those uses: Unless you have a wide range of temperature control, a 370 degree Celsius soldering iron runs the risk of burning plastic and heat-shrink tubing. Instead, you can use a low-temperature hot glue gun, this will run at 120 degrees Celsius and therefore will be within the temperature range of heat-shrink tubing. A standard temperature hot glue gun is within the range of 200 degrees C, and therefore can be used to weld plastic together.
20191118/DuckDuckGo low temperature hot glue
20191118/https://www.gluegun.com/blogs/tips-tutorials/8349592-low-temperature-glue-guns-glue-sticks-guide
20191118/https://answersdrive.com/what-temperature-does-hot-melt-glue-melt-at-676969
On the other hand, if you want to be more daring, you could use a small fan and a soldering iron to form a small makeshift heat gun that would probably be within the temperature range of heat-shrink tubing. Or, as Wikipedia recommends, you could simply hold the soldering iron very close to the heat-shrink tubing without touching it.
20191119/https://en.wikipedia.org/wiki/Heat-shrink_tubing