The Thunderbolt 3 port is also a USB 3.1 gen 2 USB Type-C port (well, it is a USB Type-C port supporting USB 3.1 gen 2, and has Thunderbolt 3 as one of the supported USB-C alternate modes). You need a USB-A female (receptacle) to USB-C male (plug) adapter, which can connect your printer’s existing USB cable to your computer. The adapter should support USB 3.1 so it can be used with all USB devices, not just USB 2.0.

Or you can replace the printer’s existing USB cable with a USB-B male to USB-C male cable. Check the printer’s end of the cable to see if it is USB 3.0 or USB 2.0. That will determine which kind of USB-B male connector you should get. I believe a USB 2.0 Type-B plug will connect to a USB 3.0 Type-B plug, but it will not support USB 3.0 speeds.

If your printer has an ethernet or wifi connection ability (to allow connecting it to a network), then you should use that instead, so that you don’t need to connect it directly to your computer. Your computer will be able to find the printer on your local area network (LAN). You should have a router in your house. Your cable modem or whatever device you use to connect to the internet (WAN or wide area network) might have a built in router or maybe built in wifi router. Otherwise, you can buy a router which connects to your internet device on the WAN side, then all your computers, tablets, printers, etc connect to the router on the LAN side. This way, all the devices can use the printer.

If the printer is connected to your computer instead of the network, then your computer can share the printer so other computers can use the printer, but your computer needs to be on to do that. That’s why a network connection for the printer is preferable.

The Thunderbolt 3 to Thunderbolt 2 adapter does not pass DisplayPort or HDMI signals.

You need to use USB-C alt mode adapters instead of Thunderbolt 3 adapters for HDMI, VGA, etc. You can use a USB-C to mini DisplayPort adapter, then connect your adapters (which are mini DisplayPort, not Thunderbolt) to that.

Yes, sort of. You need a USB-C to USB-A cable, and then you can provide a very low voltage to the laptop like it would to an iPhone or iPad. in my tests it only works when the laptop is asleep, but if you leave it overnight, it will be fully charged when you wake up

Yes, the Apple support document HT207266 “Connect devices and displays with the Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter” says you can do that.

I don’t know how fast Thunderbolt networking is in that case. The networking packets might not be allowed to use the full 10 Gbps bandwidth. For example, Thunderbolt 3 can transmit 40 Gbps, but PCIe transmission is maxed out at around 22 Gbps. I don’t know if network traffic falls under the PCIe limitation. DisplayPort traffic is separate and can use up to 32 Gbps on Thunderbolt 3. You can read about that in Thunderbolt3_TechBrief_FINAL.pdf at thunderbolttechnology.net

The macperformanceguide website has a very good review of Thunderbolt networking “Easy Thunderbolt Networking (10 Gigabit)”. Those tests used two Thunderbolt 2 Macs to achieve over 1000 MB/s but that occasionally dropped to under 100 MB/s. I guess Thunderbolt 1 would be half as fast. I don’t know if Thunderbolt networking has improved in the last two years.