Database update removes all customization

When you say “customize a theme” do you mean via the theme options, or by editing theme files. If the latter, you should never edit theme files; always use a child theme to make modifications.

When you say “database update” do you mean the database update that is necessary when WordPress itself updates? If so, that shouldn’t affect any of your data. You should be backing up your database on a regular basis.

If you mean a database update that your host did, such as upgrading your MySQL version, you would need to contact them.

Can’t advise further until we’re sure of what kind of update you are talking about.