Bitcoin node and Electrum server are 2 different thing. Bitcoin node refer to a device that run full node software (such as Bitcoin Core) to store blockchain, relay data and perform verification. Meanwhile Electrum server connect to full node to get blockchain data and index it, so it can give data wanted by Electrum clients.
I still have one question though; When i use the Electrum wallet (client), i can see, at the beginning that
multiple nodes are "synchronized" (for redundancy and safety?). In case i run my own bitcoin node, i guess i will
ask Electrum to rely only on my node (for privacy).
So here is the question:
Should i understand that redundancy is mandatory for nodes ran by strangers, and that since i trust my own node, i don't
need redundancy? I'm apologize if i'm not clear...
Yes, you should configure Electrum client only connect to your Electrum server if you care about privacy. If you're interested with why exactly Electrum client connect to multiple server, read
https://electrum.readthedocs.io/en/latest/faq.html#does-electrum-trust-servers.
Also correct me if i'm wrong, but there is no rewards in running a BTC node, although it is not the case for the Ethereum network.
Yeah, there's no financial reward a BTC node. Although on ETH network, IIRC you also need to be validator or sequencer to get financial reward.