If you do not want Capture One start with the default Catalog, press the Option key (macOS) or the Alt key (Windows) whilst launching Capture One. The Recents windows will appear so you will be able to choose the preferred Catalog/Session or create a new one.
After installing and launching Capture One 20 the very first time, the software opens with a Catalog. This should be the standard behavior for new customers who have just started using Capture One. However, existing customers may also experience the same.
NOTE: This is a known issue in Capture One 20.0.0. Please download Capture One 20.0.1 where this issue was fixed for Mac version.