I have the anthology through GOG, but wanted to try them on my phone. Which file(s) do i need to upload, the one called "_ZORK1" looks suspiciously small?

While I admit I don't know anything about loading them onto a phone, I will point out that this is an all text adventure game from about 1980. It's file size will probably inevitably look suspiciously small to anyone used to the games of even 20 years ago. At the time, the whole game had to fit on a single 5.25" diskette.

I'm not trying to sound condescending here, but you should know what kind of games you will be getting if you play most of Infocom's old catalog. Even the few games with substantial graphics at the end will be most heavily text driven with stationary graphics. My view is that they still might be worth playing, though, of course, that's partially nostalgia talking, but not everyone today will feel that way.

ArthurWalden is correct, these being ancient text adventure DOS games (Zork 1 - 3) they are really small.

The file you want is in the DATA folder, called ZORK1.DAT.
The same for Zork 2, DATA\ZORK2.DAT
The same for Zork 3, DATA\ZORK3.DAT
Planetfall, DATA\PLANETFA.DAT
Zork 0 needs the ZORK0.ZIP, for the graphics you need to find newer BLORB formats. The CG1/EG1/MG1 files are not supported usually.
https://ifarchive.org/indexes/if-archive/infocom/media/blorb/
Beyond Zork, Data\BEYONDZO.DAT, as above the mg1 is not supported (this is simply the title screen) and you will need the BLORB version from site above just like Zork 0.

Note: You must rename the blb files to have the same name as the DAT files. So, ZORK0.DAT needs the blb file to be named ZORK0.blb. Note 2: BEYONDZO are the letter O not zero.

Perfect, thank you!