Use the Join one To first join operation of Add Join for a case insensitive join.
https://pro.arcgis.com/en/pro-app/latest/tool-reference/data-management/add-join.htm#:~:text=One%2Dto%2Dfirst%20joins%20are%20not%20case%20sensitive%3B%20one%2Dto%2Dmany%20joins%20are%20case%20sensitive.
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