Hey there!
First question being, how much coding / implementing does the composer actually do, and how do I practice?
It really depends on the projects. For example, there are plenty of projects where you'll just create the music and toss it over to a programmer for implementation. There are other projects where you'll be putting the music into an engine (Unity or Unreal for example) or into middleware like FMOD or Wwise. While this doesn't require straight up coding experience, being able to think like a programmer and understand how games operate is a huge advantage. You can practice with any of these with free sample projects/games and there are tons of references, free videos on Youtube as well as paid classes on sites like Udemy. Even the engines themselves have classes you can take for free.
Second question is, how do I build up a portfolio in a presentable way?
I would look up composers you admire and take note of how they constructed their portfolios. Then copy what you like about each. You can also find sample projects where you can show how your music is reacting to game states/situations to highlight your ability to create interactive music.
Last question being, what are good ways to find small games / projects to work on?
Tom is right - check out the hobbyist section! Best of luck!