i want to build website but many things i do not know about it .like the security of wordpress theme
Wordpress has a fairly rich set of base features, including security. Moreover, there is a large ecosystem of plugins, free and paid, for expanding Wordpress base functionality, like security, and do event more, such as ecommerce, learn management and appointment scheduling.
I recommend creating wirefr of what you want to do and user stories describing the functionality. Then, either research the available plugins or engage an experined Wordpress pro to assist you.
Yes. It is entirely possible. There are two ways to go about it. Hook into their internal systems to use their flexible tables to store data, or write your own custom tables and store the data there. Depending on what you are doing and the sensitivity of the data will determine which method works best for you.
For instance, I built a Yacht Broker plugin to import data from a third party system and then use it in Wordpress. I went the Wordpress Native route, and kept things in options tables. But I eventually ran into issues due to the size of data and how they wanted to query for the yachts. So I synced the data from options first, into post meta, and displayed it based on the yacht post themselves, eventually creating the ability to autogenerate posts based on the third party data.
I could have done this in custom tables and increased query performance, as Wordpress uses a taxonomy system that can be hefty if you are storing lots of data, not to mention the client then wanted taxonomy meta to be able to describe taxonomies and modify them, which I did by hooking into Wordpress yet again and storing tax meta in options section.
I even took advantage of WP_Cron as the data we were syncing was large and needed to be brought in in chunks due to PHP execution and memory limits. I did this so that a person wouldn't need to install a CRON on their server, making the plugin very portable.
The disadvantage was that WP_Cron runs when someone accesses the website. so if no one visited it wouldn't start. It was complicated, but it worked in the end very well.
So, it all depends on what you want to do!
If you are doing some kind of custom ecommerce it makes sense to write to your own tables, just namespace them correctly so there aren't any conflicts with other plugins.
I've done a ton of Wordpress Websites, themes, plugins in my career so if you need any advice please feel free to ask.