simple trim question

I just found out that in my Nikon Studio software, if I trim a 4K video, it will reduce its resolution.
I've compiled lots of videos in Resolve, but in this and other cases I just want to trim a video in its original status to replace the archive original - for example when the recording was accidentally left on and there is nothing but garbage taking up huge amounts of hard drive space.

Do I have to make a single clip project and make sure all the settings are the same and export, or is there some simple software that will do this automatically?

Try Media Management set to Trim. It can't handle all codecs, but most of them. If yours is a GOP codec, it may add a few frames up to the next I-frame.

Chapter 45 has all the details.

Do you have lots of clips that need trimming or just one or two?
If the latter, you can use an app like QuickTime - or the equivalent on a PC or Linux - to trim the file.
If lots then Media Management may be a good idea. But you've got to make sure everything you want to keep is actually used somewhere. As in used in a Timeline.

Yes, just make a rough cut and use that timeline for Media Management.
IIRC, the free Shutter Encoder can shorten clips without re-encoding.