I know this question was probably asked some hundred times, but sadly no answer that I found in here really helped me.
I tried these answers for example:
You can use built-in bootstrap class to achieve this.
What you need is the container to be a column flex container . class to use are : d-flex flex-column
To set the container to height:100% you can apply the class h-100to html, body and the container or add to the container style height:100vh;
For the footer, a margin-top:auto will do, the class to use is : mt-auto;
example below to run in full page mode
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codepen updated https://codepen.io/anon/pen/PBpgNN
reminder for boostrap classes https://getbootstrap.com/docs/4.5/utilities/flex/ about sizing see https://getbootstrap.com/docs/4.5/utilities/sizing/