How much power he wants on 12v ?
If you want to keep it super basic, the easiest approach would be to simply parallel all 8 batteries. It's not "healthy" for the batteries and you can't produce a lot of power on 12v, as you'll be boosting 3.7v..4.2v up to 12v but on the upside you'll have super easy charging from a 5v usb port or using a basic barrel jack and wallwart adapter and producing the two voltages can be done with cheap simple boost regulators.
For charging, you could use something as basic as a MCP73812 charger chip :
https://www.digikey.com/product-detail/en/microchip-technology/MCP73812T-420I-OT/MCP73812T-420I-OTCT-ND/1979800It takes in up to 6v (from a usb port or barrel jack connector) and charges batteries with up to 500mA ... divided by 8 you get around 40mA per battery, so it's slow enough to not overheat the batteries, to balance themselves out by some degree and so on..
You'll find tiny circuit boards with this chip or others in the family, like MCP74832, where you have DC input and battery connection and maybe an option to select maximum current (up to 500mA)
If you want slightly faster charging but similarly simple chip Skyworks has one that can charge at up to 1.6A :
https://www.digikey.com/product-detail/en/skyworks-solutions-inc/AAT3693IDH-AA-T1/863-1512-1-ND/4246137That's around 200mA per battery, if you have 8 batteries in parallel
It's a bit more difficult but you can solder thin wires to the pads, for example you can get a solid core ethernet cable and take out the 8 wires inside and solder the wires carefully to the pads
You can get step-up dc-dc converters which can convert low voltage to higher voltage with reasonably high efficiency.
Here's a couple examples:
up to 3$ each, up to 2A of current :
https://www.ebay.com/itm/DC-DC-Boost-Step-Up-Voltage-Regulator-3-3V-4-2V-5V-6V-9V-12V-2A-Converter-Module/322590885144 (select from drop down list 5v or 12v output, you can manually change voltage by changing resistors on the circuit board)
12v only (seems like same chip) :
https://www.ebay.com/itm/Mini-Step-up-Converter-DC-DC-3V-3-7V-5V-6V-to-12V-Boost-Voltage-Module-Regulator/112855472220up to around 2.5A of current, select voltage from drop down list :
https://www.ebay.com/itm/3-5A-DC-DC-Step-up-Boost-Voltage-Converter-Module-3V-3-3V-3-7V-to-5V-6V-9V-12V/2826306749511.2$ up to 1.2A of current, 12v out version (you can change resistors to get 5v out) :
https://www.ebay.com/itm/DC-3-3V-3-7V-5V-6V-to-12V-Boost-Voltage-Regulator-Converter-Step-up-Power-Supply/2718121054432$ : fixed 5v output, maximum 0.48A, up to 85% efficiency :
https://www.ebay.com/itm/DC-Boost-Step-Up-Converter-1-5V-3V-3-7V-4-2-to-5V-Voltage-Regulator-Power-supply/172438981311