I'll get the answers above me and all the research I did today and answer the topic. Thanks to the people who helped. Correct me if I'm wrong.
1) Modem and router: a modem is the machine that connects the house and the internet outside. A router is the one handling all the local network inside the house. Most of the devices today combine both. Google Wifi is a router only. It needs to be connected to a modem. It only helps to diffuse the WiFi better in a big house or a house with walls that can cause issues to the WiFi.
2) Internet bandwidth and sharing: let's say I pay for 100Mbps Internet, but because of the distance to the DSLAM I get only 60Mbps internet. If I plus my laptop in Ethernet I should be able to receive all the 60Mbps. If another device is downloading, these 60Mbps are shared between us, let's say as an example, 30Mbps each.
3) Dual/tri bands routers: Devices connect via radio waves. The WiFi uses the 2.4GHz frequency. If too many devices use the 2.4GHz, some of them might have somme issues (like getting disconnected). So now, some routers have the possibility to desserve WiFi through another band, the 5GHz one. It means, on the device, we will see 2 networks: home2.4 and home5 (for example). This doesn't multiply the bandwidth, means we still have only 60Mbps from the ISP. It helps to keep a better WiFi connection when a lot of devices are connected to the same band and use the WiFi.
4) WiFi type (g/n/ac) and Ethernet type (100/1000Mbps): The WiFi G can provide 54Mbps. Nowadays, everything is at least N (mostly AC even). Knowing in my example that I receive 60Mbps, a WiFi G would simply handicap me from 6Mbps. The computers have most of them the ability to receive, via Ethernet (and whatever connection there is like USB Type C), 1000Mbps. So we're still good.
5) Some usage: let's say I want to cast 4K video to the TV. The cast needs 15Mbps (Netflix says 25), so with the 60Mbps we're still well above the criteria to buy a new modem.
Conclusion: in my example, there is no need to buy another modem. Eventually, if many devices would use the 2.4GHz band I would may be require a new modem dual band.
I have a TP Link mono band router (W9970) and I would like to upgrade.
First question, is Google WiFi a router? Can I replace my router with it? It looks so good. My only concern, I assume I will need to add a switch if I take that.
Second question, I was thinking about taking a tri band router like the Jetstream AC3000 or the Linksys AC2200.
What do you recommend? I'm lost.