Retro hand held games....need advice

Buying a couple of these for kids for Xmas, may even get one for myself. Trimui smart plus and Anbernic 40XX H

Will probably receive the game units tomorrow, 64gb loaded with games. I have never used on of these devices nor know anything about them,

Would appreciate any help, compete newb

I assume you can download extra games. Just wondering where and how? Is there a website / websites to download? Is it free?

Do you need to load games specific to your brand device or do these emulators all run the same games (within device capability)?

This is supposed to be able to run some PSP / N64 games (but not the best), depends on the game apparently.

Is it ok to download some and try or will it crash the drive or something?

To back up the card that comes with the device can I simply copy the files off the SD card onto my computer?

If I wanted to D/L more games and the 64gb card is full, do I just get another card and put in slot 2 or is it better to just get say a 128gb card

and have them all on the one card

Many thanks in advance, sorry if they are stupid questions....really know nothing of these devices