They do use their infrastructure to connect to Apple Notification Service servers when the app is not used, they do act as a Man In The Middle but in a secure, concise manner (not in a sketchy way). And they conect to their servers for registration and subscription status. You can read a more in depth explanation on their blog
https://blog.beeper.com/p/how-beeper-mini-works
Besides all that I’m not sure if someone who wants to create it’s own implementation of all of this can do it without any apple device; reading jjtech technical explanation (https://jjtech.dev/reverse-engineering/imessage-explained/) where he explains pypush he mentions the obfuscation process for registering a device to apple servers, here is where pypush somehow manages to convince Apple that the machine is genuine, there is a mention there to some serial identifier stored on a file called data.plist, if someone wants to implement this proof of concept would need to give another serial identifier?
That’s just the proof of concept, there is a lot of work and infrastructure besides that code. You can read a more in depth explanation of how it works on their blog
I definetely wouldn’t trust that link; Beeper mini functionality requires you to conect to their servers for registration, initial handshake and push notifications so I don’t think there is a way to avoid a subscription
Edit for clarity: I wouldn’t trust the apk you linked in your edit, not the original Beeper mini app in the play store
Be ware of using VPN if you want spanish content and wanna join a private tracker; all spanish private tracker ban the use of VPNs
My advice is to check if your ISP is blocking you from access to the sites you plan to use and pick one that doesn’t. If you plan to use torrent check if your ISP is outside CG-NAT (or let’s you leave it) to enable port forwarding
If your country doesn’t care about piracy do not complicate your life further till they care, just stay up to date. As for measures I always use independantly of context, I just recommend using
Hosting a git instance with Njala?