On Ubuntu I always had a hard time resizing windows as window borders are so thin that they are hard to grab.
My solution for Ubuntu Mate was to resize windows with ALT + right mouse and move them with ALT + left mouse.
Ubuntu with Gnome Shell (Ubuntu 20.04) uses different shortcuts. You have to use the super key (windows key) with mouse-wheel-click to resize.
If this feels awkward to you, you can change this using the Tweaks application:
There are also some keyboard shortcuts to move and resize windows:
Super + Left/Right: Resize the window to the full height and half width of the screen and move it to the left/right border.
Super + Shift + Left/Right: Move the window one screen to the left/right
Super + Up: Maximize the window
Super + Down: Unmaximize the window
You can customize those keybindings in the "regular" Gnome settings application.