I would like to export all my VPN configurations stored in my smartphone to my pc. I use three VPN apps (VPN Master, USA VPN and Bettermet), in your opinion have I to find by myself all certificates and credentials or is there a place where I can find all this stuff stored in the system settings?
Different apps working with different VPN protocols have different methods of handling authentication credentials (username, password, certificate, key etc.). I didn't use any of three apps you mentioned, I guess those are all proprietary apps with purchased services (or at least under some T&C; if you don't pay them money, they charge you some other way). If you have root you can check their data directories under
/data/data/, but I don't think you would be able to extract any useful data from there. There would be encrypted files, though some (free) VPN apps save VPN configuration (e.g.
.ovpn) files in plain text.
Some general purpose apps like OpenVPN Client - the one I use with my personal OpenVPN server on AWS EC2 - have a UI option to export VPN profiles as text files. But that won't be the case with purchased VPNs if they don't want you use the service on multiple devices or particularly on PC. You may ask them for a multi-device license / contract or go for some other option. https://www.vpngate.net, for instance, is a network of volunteers who offer a large collection of VPN configurations of multiple protocols that you can freely use on any number of devices.