How can I trim silence from all regions in a single track at one time?

drag click and select the regions you want (or hold shift and click the ones you want to select). If you can to cut them all at the same time point (e.g Bar 4 of the track), move your play head to e.g bar 4 then press Cmd+T. T is trim :) then select the end of the regions and delete them.

If you want to shorten them all by the same amount of time but they're not all at the same time mark, just drag them by the end of the region to make them smaller :)

Assuming you’re happy with your edits, the easiest way is to select all regions, then join them (“J”) - making one continuous region - prior to Trim Silence (ctrl-X). (You might want to copy the regions to another track, so that you can revert if any of your edits are off).

Be sure not to make the trim edits too tight to the audio - better to leave a small gap then add fades. (If you’ve a lot of background noise, eg. headphone spill or guitar amp noise, it can sound a little abrupt without fades).

That being said… sometimes the manual approach (region by region) is often better, as you can see more what trim silence is actually doing. With shortcuts, it’s often quicker than you might imagine too :)