GlassWire - Visualize network activity in detail, get notified when new apps access the network, look out for...
Nethogs - NetHogs is a small 'net top' tool.
TinyWall - Lightweight and non-intrusive firewall
Portmaster - Is a Privacy Modular Privacy App which includes a "DNS over TLS" feature as well as an "Application Firewall".
Little Snitch - Little Snitch is a firewall application that monitors and controls outbound internet traffic.
nload - Monitor network traffic and bandwidth usage in real time