2 Network Adapters of PC connected to same router, how does the connection work?

2 NICs connected to same router. Speedtest 1: When connected via ethernet only, I get 100Mbps up/download speed. When connected via WiFi 5GHz only, I get 300Mbps upload (capped by my internet service), 150Mbps download speed. Speedtest 2: I look into my router's interface statistics table (Mikrotik HAP AX2). When 2 of them are connected at the same time to same router, I get varying connections active at one point. Sometimes the WiFi is active on both upload and download test, sometimes WiFi is active during download, then ethernet connection is active during upload, and sometimes only the eth

Question by AgapitoFame on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.