I love the “bigclive” YouToube channel: it is full of neat ideas, and wise tips on disassembled electrical gadgets. Most of his videos involve soldering or/and making schematics of the things he tears apart or builds.
Modern LEDs are bright enough at low current to use as directly mains powered indicators with minimal circuitry. The slight downside is that with a simple resistor circuit the vast majority of the power gets wasted as heat. But the current is so low anyway that it doesn’t really matter. Here are some simple ways to drive LEDs from 110V to 240V with minimal circuitry. I tend to recommend running resistors at around half their rating at worst as it means they will last a long time and not discolour too much with age.
His [WayBack] bigclive.com web-site is very small and shows some of his projects.
It actually started as a Dutch site, is ran by a Belgian now living in Barcelona. This means many articles are available in various languages, not just English and Dutch, but many also in French, German, Spanish, and Italian. Some articles are only available in English or Dutch.
[WayBack] Yepkit – Electronics: YKUSH Yepkit USB Switchable Hub (models for USB 2 and USB 3.1 allowing to switch 3 ports at maximum)
[WayBack] Robot Electronics: Relays (both mechanical and solid state) in many communication kinds (ethernet, USB, RS485, TTL, Bluetooth, Modbus, MIDI and DMX) and sizes (from 2 to 24 ports).
From the picture linked above (full one at the bottom of the post): a switch is missing from the parts list (:
Note that this is only the hardware parts, and no measurements of the voltage from the Raspberry Pi. So it is a battery backup, not a UPS.
For a UPS kind of functionality, you need more hardware (measure voltage to assess when to safely shut down, reset circuitry for when power comes up), and software (so the devices can shutdown).
Connections
After IRC chatting with Lewis (see his [WayBack] profile), this is how they should be connected:
Most of the wired “parallel” 18650 dual battery holders on non-USA Amazon sites are in fact serial (you see that in the comments, or by watching the wiring carefully), so better buy an unwired one!
Connections to the adjustable LM2577 step-up converter having a max current of 3A
(get one *with* voltage meter, as voltage meteres themselves are about half the price of the LM2577!)
IN pins from the TP4056 OUT pins
Adjust to slightly above 5V before connecting anything else!
I just set up a rpi 4b (8gb) w 256gb ssd via USB 3. Ubuntu 20 64bit. Booting from USB still not quite there yet, and of course it takes me days to get the silly thing mounted. Btrfs on the SSD.
[WayBack] GitHub – dschmenk/apple2pi: Apple II client/server for Raspberry Pi: hybrid computer of a Raspberry Pi inside an Apple II (either ][, or ][+, or //e) so the Apple II can be a front-end to the Raspberry Pi which then can run an Apple IIGS emulator, talk to the Apple II storage hardware and much more.
If you want to measure both voltage and current, then a combined piece like 
The Wiert Corner - irregular stream of stuff 
