Removing more than one part
Most online cutters ask you to pick one region and keep it. That works for a ringtone and for nothing else. If you are cleaning up a recording you usually want the opposite: keep the whole thing but take out the four places where someone coughed, the phone rang, and the meeting drifted.
Select a stretch and press remove, then do it again somewhere else. Each removed stretch appears in the list under the waveform with its exact times, so you can correct one without starting over. What is left is joined in order.
Where the cut lands
Cuts here are sample-accurate, not frame-accurate. The audio is decoded, the pieces you kept are joined, and the result is encoded fresh — so a join lands exactly where you put it rather than snapping to the nearest compressed frame, which for MP3 would be up to 26 milliseconds away.
The trade-off is that trimming re-encodes. If you only need to shorten a file and do not want to lose any quality, extract to M4A instead: that copies the audio untouched, at the cost of the cut snapping to a frame boundary.