Help Needed with KDE Plasma Customization

I’m new to KDE Plasma and I’m loving the flexibility and customization it offers. However, I’m having a bit of trouble figuring out some aspects of customization and could really use some help from experienced users here.

Here are my specific questions:
Panel Customization: I want to create a custom panel that includes specific widgets and shortcuts. How can I add and arrange widgets more efficiently? Is there a way to lock the panel once I’m happy with the setup?

Theme Installation: I found a few themes online that I’d like to try. What is the best way to install these themes? Are there any recommended repositories or websites for high-quality themes?

Window Management: I’m trying to set up specific window rules (like always opening certain applications on specific virtual desktops). How can I configure these rules in KDE Plasma?