These warnings result from the fact that you are using PHP8 and many plugins have not yet been adapted for PHP8
However, here in the forum you can already find adjustments for many plugins for PHP8 in the respective support threads for the plugin
Alternatively, you can deactivate the display of warnings in the ACP
However, the third warning with $unreadreports is because your templates are not up to date; In this case you have to open the template header in the ACP and search there
Find $unreadreports
and replace with
Replace $modnotice
Since this change is already very old, there is a risk that other templates in your board are also not up to date, so check in the ACP under Templates --> Find Updated Templates