Disk Utility on macOS Catalina 10.15.7 does not let you shrink or extend the Bootcamp Windows partition.
There are lots of third party software around, but most of them did not support Catalina OS the time of writing (April 2021).
I found a working software: Paragon Hard Disk Manager. The demo version seem to fully work for free for 10 days, so keep that in mind.
It took some time to install, asking for admin permission to install two system extension, but then worked like a charm.
The interface is very clear and obvious. Just right click on the partition and select “Move/Resize”.
And from here you can choose to change the partition size. Move the sliders to easily fill all the empty space, shrink, extend or move it.
Click Apply operations and wait. It correctly moved for me a 40Gb partition in only a few minutes
Enjoy, and clap if useful.