For servers: collect sequence numbers, types, etc. Collect the number, frequency and type of CPU; The number and model of GPU cards Hard disk, memory, network card is similar; OS version, etc.
For network devices: snmp management, collect device version, model, sn, etc
For servers - you'd need to install netxms agent and set
MasterServers = IP-address-of-your-netxms-server
in nxagentd.conf
After adding a node in netxms, it should detect that agent is present on that system, then you can add DCIs (data collection items) for the metrics that you need.
Also, check the docs, there's some info there:
https://www.netxms.org/documentation/adminguide/concepts.html
https://www.netxms.org/documentation/adminguide/quickstart.html