You cannot join any beta without the container app. Go to the official Apple App Store and search for "TestFlight" (developed by Apple). Download it for free.
When a link drops, it fills in minutes. Have TestFlight installed, your Apple ID logged in, and click the invite link immediately. No hesitation. youtube beta testflight free
If you manage to get a link, ensure you have a spare Apple ID. Beta builds can be unstable. Installing on your main ID might leave leftover profile data. You cannot join any beta without the container app