Un-tit-led

Mobile Gaming is really bad for your health and should be completely avoided at all costs. More details here
Most Emulation requires some sort of mobile gamepad.
Very few touch games on Switch/DS/3DS emulation can work without it but the games are very little
Do consider Handheld Consoles which the cheapest ones might not be much more than the gamepad above and can be a dedicated device in itself.

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Emulation on Android

Just  spend a few days with emulation and gaming on my terrible phone (Iqoo Z9s Pro 5G with 7Gen 3 that heats up quickly to 44°C ) and here's some findings:
1) Eden Emulator is better for Switch Emulation however your phone should stay relatively cool otherwise don't bother. And even then it's mainly 2D games or very basic 3D Games that work fine. Again, this needs testing just like everything else.
Citron crashes with any sort of keyboard input. Citron is best for Switch. All others give worse performance, so much worse that I thought to give up Switch emulation at all.
2) Citra is best for 3DS. Didn't bother with others, didn't need to.
3) PPSSPP is best for PSP. Didn't bother with others, didn't need to.
4) NetherSX2 patched based on AetherSX2 is best for PS2 emulation.
5) Stay away from Windows Emulation. Find the same games on switch and play them there. In rare cases like very old games that are only on Windows might work but so so so so so many issues that you might as well give up.
6) Many unofficial ports of many games exist but apart from a few they are all buggy.

Should probably put this on the website somewhere.

Emulation on iPhones

Don't own one and won't look into it unless people are genuinely interested.

Cautions

Gaming Blogs

@Archived

Emulation on iPhones