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To improve cross-device attribution, it’s necessary to implement the tracking of the user ID in Google Analytics.

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For GA4 via Tag Manager

1

Pass the parameter to dataLayer

On every page load after login, pass the relevant parameter user_id to your dataLayer.
2

Create a GTM variable

Create a Variable inside GTM that reads that value from the dataLayer.GTM user_id dataLayer variable
3

Send the property to GA4

Under the Configuration Parameter section of your GA4 Google Tag, add a new row with the user_id property and the corresponding Value (the Data Layer variable you created in step 2).GTM user_id config parameter

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For GA4 using gtag.js

Pass a unique user ID after login to track users across sessions on different devices using the user_id config parameter.