I know this is pretty much a duplicate of Can you hide attribute columns in QGIS? but I have about 100 columns I want to hide so right clicking each column - Hide is not ideal.
Right click on a column name, as you do to hide a single column, but choose "Organize Columns..." instead.
Click "Deselect All" (The Select All and Deselect All buttons only appear if you have a lot of columns, otherwise you have to untick them individually).
IMPORTANT: Tick at least one column back on.
If you don't tick one column back on then you can not get any columns back:
At least until the fix is live:
Fix has been merged into Master, so should be in the next version of QGIS 3.14. Should be backported into 3.10 LTR, and possibly 3.12.