Everywhere write about plugins for WP. But can we do without them?
It is possible and even necessary.
Ready-made plugins-slider WP - infernal trash and waste. There is usually a Adobe Premier and Adobe AfterEffects in the admin panel that lets you configure the transition effects between slides, the duration (and even Bezier curve), animations and so on. A reasonable person, this heresy is of course not necessary.
To output uploaded images in your slider you just need to get the right attachments (post type) query WP_Query. How to organize the loading mechanism of pictures in the admin panel in the desired location (Customizer settings page, the edit page, front page, etc) - depends on where you need it, but the easiest and most convenient way is to use the ACF plugin (or equivalent).