How can I create a specific template for all posts of a category ?

Hi there,

to display different menus for different pages/posts etc. requires a plugin like this:

https://en-gb.wordpress.org/plugins/conditional-menus/

I would like to use it this category navigation as left sidebar and it seems not possible. It would be possible if I could change the widget on specific pages. Is it possible ? Thanks

You can use the Secondary Navigation option in GP and set its Location to Left Sidebar:

https://docs.generatepress.com/article/secondary-navigation-layout-overview/

I want to be able to have different secondary menus and assign them to specific category pages. I can create more menus with the plugin you advised but it is not possible to manage different menus with the GP secondary menus. Same issue with widgets that are limited to one par position. I was thinking of using different templates through the site but I see it is not possible with GP ... Maybe with elementor pro ??

I am not sure what the issue is.
That plugin should allow you to create multiple menus all of which are assigned to the Secondary Navigation, each with their own display rules.

You may have a problem ( not sure ) if you're using a Navigation Widget to display the menu. But if you set the Secondary Navigation Location to Left Sidebar it should work.

All this seems confusing to me - should I use widgets or elements since elements can be displayed without widgets ?

Can you try setting the secondary navigation location to left sidebar in Customizer > Layout > Secondary Navigation instead of using navigation widget?
https://www.screencast.com/t/oUfcROmdoM