Hi @Yuval Simon welcome on the community. It depends always on the implementation of the plugin.
If you need more than getting custom field value, it must provide some REST API which should be documented.
For example https://marketplace.atlassian.com/apps/1219732/time-in-status-for-jira-cloud?hosting=cloud&tab=support only let you to get custom field value (or I didn't find a documentation) -> https://saasjet.atlassian.net/wiki/spaces/SAASJET/pages/1533542401/How+to+get+SLA+data+via+Jira+API
This plugin - https://marketplace.atlassian.com/apps/1211756/time-in-status?hosting=cloud&tab=support has some reporting REST API implemented (https://dev.obss.com.tr/confluence/display/MD/TiS+Cloud+-+REST+Reporting)
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