Hi Henk,
Template Options - Layout (list of default positions)
and
offcial guide : https://www.joomshaper.com/documentation/helix-framework/helix-ultimate-2/layout-builder
Your Questions - My answers
which module position can I best put it on
It depends where you want to display it? For example if above component "content-top" , if below "content-bottom" or "bottom1...4"
ICAGENDA plugin
If template module positions you can publish only modules, not plugins itself.
In general iCagenda is component with modules in package, so you have to choose judiciously
but you have a choice what display view use and where.