AntennaPod is free, it is developed by volunteers. Note that the large platforms usually give nothing of your subscription money to the podcasters. Usually (unless is is exclusive shows) you are paying only to the platform, not to the podcaster. The podcasters publish their shows for free and the platforms charge you to show their free content. For AntennaPod, the platform is free. Podcast creators can create paid subscriptions or add sponsored segments, but that is the same for every platform, paid or not.
We would definitely appreciate you donating to your favorite podcasts! Most have a donation link somewhere on their website.