After the 30 day trial, your SketchUp Pro licence will stop working.
However, you won't lose any work that you have created. You can always load them in the very basic free version.
If you buy SketchUp during your trial, you will see your licence become active as soon as the trial has finished.
If you buy SketchUp after your trial, you will be able to sign in with your Trimble ID. Just make sure that you purchase with the same email/Trimble ID that you used for the trial.