Hello, thats a lot of questions I hope I can answer them all fulfilling your needs.

1. the IoT2050 itself can replace a PLC or not?

No, the SIMATIC IOT2050 is designed as an industrial IoT gateway and open industrial PC-like platform, not as a replacement for a PLC. Its main functions are data collection, preprocessing, and forwarding, as well as running computer tasks with low performance requirements

1a. if yes - how to connect relay's coils or relay's feedback contacts? 

Not applicable, since IoT2050 is not designed to directly control relays as a PLC would.

1b. if no - it shall be expanded by the SM- modules (up to 6) but which ones (instead of connection to an existing PLC)?

Currently the IOT2050SM supports up to 6 S7-1200 modules at once. The following modules are supported at the moment: 6ES7 223-1QH32-0XB0, 6ES7 223-1PL32-0XB0, 6ES7 231-5PD32-0XB0, 6ES7 231-5PF32-0XB0, 6ES7 231-4HF32-0XB0, 6ES7 238-5XA32-0XB0, 6ES 7647-0CM00-1AA2, 6ES7 231-4HD32-0XB0, 6ES7 221-1BF32-0XB0

2. the connection through a vpn is possible with the device alone?

Yes, the IoT2050 runs a full Debian-based OS, supporting standard VPN solutions (OpenVPN, WireGuard, etc.) as available in the Linux ecosystem. You can install and configure VPN packages using standard tools

2a if yes - how to administrate vpn, firewall and functionalities?

The administration is depending your used tools for vpn, firewall etc. In general you can use command line and configuration files for the standard debian tools. There are enough guides in this forum or in the internet.

2b. if no - can it be done via CloudConnect7 + ScalanceRouter? Or is there a PLC (S7) necessary?

not necessary as you can use IOT2050's own VPN capabilities.

3. If connected to a PLC - is it possible to connect it to any PLC (Beckhoff, Selectron, WAGO) and what are the limitations if not a S7 is used?

The IoT2050 is an open platform supporting many protocols (Modbus, OPC UA, MQTT, etc.). It can connect to various PLC brands, provided protocol compatibility is ensured. Siemens S7 PLCs offer the deepest integration and support, but other brands can be connected via supported protocols. Limitations may include lack of dedicated Siemens application examples for non-S7 PLCs and possible manual configuration

4. As the support is only through this forum - are there any training available from Siemens or 3rd parties?

Yes there should be trainings available at SITRAIN Digital Industry Academy.

5. Is it possible to use an open operation system (OSS)? which ones? what are the limitations of using OSS instead of a SIEMENS-  Example_Image?

Yes, the IoT2050 supports Debian Linux, which can be freely installed and customized. Other Linux-based distributions may be possible, provided hardware compatibility. The main limitation is that Siemens' Example_Image and IOT IndOS includes preinstalled drivers, protocols, and industrial connectivity features, whereas a generic OSS install may require manual driver and protocol setup. You can also build your own image based on the IOT2050 Example Image: https://github.com/siemens/meta-iot2050

BR