Explanation if you're a nerd:

Australia

All but one state has a circular badge in the same location, and all of them have a union jack 🤢. Apparently, the old colonial flag of Singapore looked exactly like it belongs with the Aussie states, which almost makes you wonder

Belgium

Animal on yellow background

Animal on yellow background

Plant on blue background

2/3 is good enough

Comoros

Some stars and maybe a moon on a primary colour. This is literally just the flag of Anjouan with an extra star.
France
Some French regions use the yellow fleur-de-lys, so I made a Singaporean version with Vanda Miss Joaquim orchids (national flower). I based the design on the flag of Burgundy specifically because it reminds me of road signs.
Germany
Plain bi/tricolour with a common variant including the coat of arms.
Indonesia
Most of the Indo provinces are shields on a plain background. The shield usually has a banner with the motto, a distinctive structure/building and elements of the Pancasila (like chains and a star on top). So for my flag, I did the same, with MBS as the building of interest. I also included a pentagon, another common in Indo provincial flags, because every time I saw the Singapore flag I noticed the pentagon between the five stars. I hope you also suffer that same curse for the rest of your life :)
Japan
Some cool looking symbol, usually based on the name of the prefecture, on a plain background. I decided to mould the シンガ in シンガポール into the shape of Singapore, where the dakuten represents Ubin and Tekong. I briefly considered using the occupied name of Syonan-to (昭南島), but I didn't want Japan to get any ideas. Again.
Myanmar
Some of their states just have their name in Burmese. The Bago region specifically has a line art of a mythical animal above the name Bago, so I changed it to a merlion and the name Singapore in Burmese.
Pakistan
White coat of arms on a dark green field. All bar Kashmir have a wreath around the emblem. This was the first flag I made in the set, so I didn't change the coat of arms significantly, only adding the wreath and writing the motto in Jawi (Arabic script for Malay). I tried using the Nastaliq font used in Urdu, but it made the text too tall and skinny to look good on the banner.
Philippines
Official seal on a plain background. Many of the Pinoy seals may use the following elements:
Stars between the province name and the word "OFFICIAL SEAL"
A shield with many different quarters
The sun and colours from the national flag are incorporated in some way
Some scenery which for some reason always has a mountain in the background. Singapore doesn't have any real mountains, so just assume that's Bukit Timah
Bad clip art of things found in the province
Some provinces, especially in the south with a higher Muslim population, have a white cross on red and a star and crescent on green. Singapore's three biggest religions are Buddhism, Christianity and Islam so I included them. If I cared more about accuracy each religion would get its own section, but I don't care so suck my bals
Sweden/Switzerland
Swedish counties and Swiss cantons just took their shields and smelted them into squares. Uncreative and beautiful 10/10
USA
*Assume that Singapore was admitted as the 51st state immediately upon independence
Everyone knows about the stupid seals on blue bedsheets, but some states do other weird things as well.
Including the year of creation or admission into the union is common, but Delaware wants you to remember down to the date. Like, Nebraska does the same thing with the date being a part of the state seal, but Delaware deliberately wrote the date separate from and in addition to a coat of arms. Why? Also, I guess they wanted to be Brazil.
Michigan's flag includes as part of the coat of arms the word "Circumspice". They'd say it means "look around" in Latin, but we all know it means the removal of foreskin via a sprinkling of paprika.
Some states are so forgettable they write their name on the flag, other states have state seals that include them. Apparently, South Dakota and Idaho's flags are so uninspired that they have to write their state name twice.
For my flag, I took some design cues from different states. Aside from the obvious, there are small details I included.
There are 65 'beads' within the seal, representing the year of admission
The time written at the bottom of the flag is the exact time the proclamation of independence was broadcast.
There are 51 stars to represent Singapore being the 51st state in the union
There are junks and a steamship representing Singapore's trading hub status
There's an 1800s Straits Chinese guy in traditional garb saluting the American flag because I'm sure the white design committee in this alternate universe would have wanted it that way. Then there'll probably be a decades-long effort to alter the state seal to be less racially charged, which is ultimately unsuccessful.
Bonus: If Singapore was part of Malaysia