Will I lose data or folders or pictures when I install a new operating system?

To answer your answer you question, No you will not lose any data if you update it is optional to backup the systems but updating will not erase your data

Must sure to keep your device plugged in when updating

Hope this helps :D

Point # 11 - Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. 

Point # 12 - For addition purposes - two Tested Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Always assume there is risk and the data may be lost. This means as @P. Phillips correctly mentions having a backup is crucial for minimizing the risk of data loss. The more important the data, the more backup copies you should have. FYI, 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. There are a lot of ways to lose data just through the normal daily use of a computer.

Normally if everything goes well a macOS upgrade should not cause any data loss. However, if something goes wrong with the upgrade, then it is possible you may lose your data (usually just temporarily, but sometimes permanently).

You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.