That's up to the app designer - they can make windows resizable, or not.
Since you're new to Mac... To resize, hover the mouse/trackpad pointer over the edge of a window. If it changes to a double-headed arrow, you can click and drag to resize. Hover the pointer over a corner of the window to reveal a diagonal double-headed arrow. This allows you to click-and-drag diagonally to resize in two directions simultaneously.
You can also try full-screen mode. Click the green button in the upper left corner of the window. That will bring the window to full screen, hiding the Menu bar at the top of the display, and the Dock (row of app icons), regardless of where it's located. To access the Menu Bar, push the pointer to the top edge of the screen and hold it there about a second. When the Menu Bar is revealed, you can again click the green button at the top left of the app window, which will return the app to normal viewing mode. To reveal the Dock, push the pointer to the edge of the screen where you've located the Dock (can be at the bottom, left, or right edge) and hold the pointer there a moment.