I've found blogger to be a bit tricky to add custom code to, but you have a few options as long as everything you're trying to add is client side (HTML, CSS, JS).
If you go to your blogger account and go to layout, you can add a widget for JS/HTML. From here you could add custom scripts if you wanted. Little things that I'd test in jsfiddle first.
You can also go to the theme option. From here, first I would press backup/restore in the top right corner and download your theme so you don't catastrophically mess something up. After that you can edit your theme. One option is to add custom CSS in the advanced section of the theme editor. Otherwise you can access the HTML/CSS directly in the blogger editor. This editor is a little rough, but Ctrl+f will help you find what you might want to edit. Using this editor + the right click inspect element on the actual blog have helped me make my blogs into something much more unique and my own.