Login to your WordPress Admin area
Go to Appearance
Go to Menu
Select the pages you want to add to sub menu
Click Add to Menu
Now, drag these menus under desired menu button
Click Save Menu
That is all. Now, you go ahead to visit your website to check your sub menu.
How to add / enqueue bootstrap CDN to WordPress. How to enqueue bootstrap to WordPress.
Get Bootstrap CDN or Download it
https://getbootstrap.com/docs/4.0/getting-started/introduction/
How to Align WordPress Logo in Center
How to style WordPress Logo in Center
Link:
WordPress Custom Logo Function
Custom Logo
How to Add Custom Logo to WordPress Theme. How to align WordPress logo in the center, on the right, and on the left.
Get the code from WordPress Developer Functionality page.
https://developer.wordpress.org/themes/functionality/custom-logo/