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Google Podcasts finally lets you add custom RSS feeds
On the mobile app, head over to Activity, and then tap on the Subscriptions tab. Next, select the three dots in the top right corner of the screen and tap on the "Add by RSS Feed" option
If you use the Google Podcasts web client, click the menu at the top left and then choose "Add by RSS feed"
Once you add a custom feed, the podcast will show up alongside your other subscribed shows