Typing HTML code in a wordpress post

…as soon as I switch to the Visual tab, the HTML code I typed is removed.

How to prevent that from happening?

Do not switch from one to the other, and no, there is no other solution. Those two editors each function independently and in their own ways exclusive of each other without ever knowing the other even exists.

But even if I don’t switch from one to the other, it remains a problem because I edit an existing post, the default Tab is ‘Visual’.

I find it unreal that one cannot type HTML in a WordPress post ?! Is there a plugin that allows doing so ?

You can type HTML into a post, but only by using the “Text” mode editor. In my own case, I use the text editor at the Dashboard and then later use the “Slash Edit” plugin to get back there from out on the page. So if you are clicking “edit” on the page and ending up in the wrong editor, try using the “Slash Edit” plugin as long as the page had first been saved in the text editor at the Dashboard…
https://www.ads-software.com/plugins/search.php?q=Slash+Edit

Overall, here is more on this entire matter:
https://www.ads-software.com/support/topic/how-do-you-prevent-the-wordpress-editor-from-stripping-out-certain-html-tags?replies=8