HTML CSS Practice - using basic CSS, CSS Flexbox, CSS Button Creator

I am still new to HTML & CSS and I have only learned CSS Flexbox for this project for the first time. I have some questions that will be appreciated if someone can explain this or point me to the right direction:

• How to master the use of flex container's width and its flex items width properties (static vs dynamic use)
• Creating variables to change one variable instead of multiple variables manually
• Alternative for IDs in HTML so that the code can be streamlined or compacted to avoid unnecessary codes and save time
• Any tips to 'automate' the HTML/CSS process as it feels like the process is pretty tedious.