Whenever macOS is upgraded to a new major version of macOS the old version of macOS is removed including the installer app which was located in the Applications folder.
Make sure you remember to empty the Trash. Check your Downloads folder for items to delete. You can also store large files on an external drive. If the files stored on external media are important, then make sure to have a backup of the external media as well as your main system drive.
Make sure all of your backups have successfully transferred to your external backup media, otherwise the drive will be full of system snapshots that can take up a lot of room.
You can use OmniDiskSweeper to locate where the largest items are being stored. You can only move or delete items located in the User folders.
Make sure to have frequent and regular backups. It is impossible to recover accidentally deleted data from an SSD after the Trash has been emptied plus an SSD can fail at any time without any warning signs. In addition if a 2018+ laptop which uses the T2 security chip ever has a hardware failure, then you may lose your data since Apple or an AASP will be unable to recover data from its SSD although an expensive professional data recovery service could attempt data recovery.