How do you parent many bones at once in a single step ?

Hi all,
this is probably basic, I apologize for this but I have spent hours trying to figure out.

Quite simple: I have >100 bones + a single “base bone”, all unparented and belonging to the same armature. How do I parent the >100 bone to the “base bone” in Edit mode?

Thank you

Select the 100 bones, select the Parent Bone Last, ctrl+p, Keep offset.

Great, Thank you thedaemon.
One more question: for insertion of keyframes, say LocRotScale, for each of these >100 bones, is there a way to do that in a single step or do I have to press “I” for each of them. I know that I can do automatic keyframing once I have pressed “I”, but what about the first time?
Hope this is clear.
Thank you

Select all, press I. Or you can use the Whole Character Keying Set and press I with only 1 bone selected, I think.

yes but it did’nt work for me at first. This is because I was in automatic keyframing with “Available”

This is not compatible with the method you described and you need first to inactivate Automatic Keyframe and use LocRotScale for instance instead of Available.
The go back to your usual setting.