I really want a superflat seed with a ton of useful ores which are easy to find. How do I create a seed, and what programs do I use?
Seeds are not “created”, they are found by trying them. The seed is called that because it is the seed value for a pseudorandom number generator which is then used to determine all of the attributes of the world; it is computationally infeasible to design a seed to produce a desired result.
Type something for the seed, see if you like the world, and if not, try again with a different seed. Or look for seeds other people have tried; there are many web sites which claim to provide such information.
You may be looking for a superflat preset instead of a seed (those have seeds, too, but are not affected as drastically).
If you choose the world type "Superflat" you can select options - there are some presets, but you can in fact change it to your liking. Here is an example preset from the Minecraft Wiki page:
A preset consists of 3 things more or less, separated by semicolons:
A list of block layers, starting from the bottom. Multiples of the same layer can be shortened by putting the number and the
*before the block name. Blocks are specified by their names and are comma separated. In this preset, there is one layer of bedrock, then 230 layers of stone, the 5 of dirt and one of grass.
The biome used for the superflat world - you can only choose one as far as I can tell, here it is
minecraft:mountainsfor that nice bluegreen.
A list of structure generation options: here, most are self-explanatory, like
dungeon, but special mention goes to
biome_1which means biome specific features like witch huts in swamps. This also must be on for desert temples (as well as having the biome set to
minecraft:desert, of course), but desert temples also require the dedicated
desert_pyramidoption to be enabled.
More documentation can be found on the corresponding wiki page, but it is not fully up to date, and some documentation is not fully clear (for example, it mentions that structure options used to have parameters before 1.13 which got removed, but other parts of the page imply that the structure options are ignored altogether, not just those parameters.
Furthermore, if you do not feel like tinkering with the format, there are apparently superflat preset generators available online. I will not link any specific ones here as they seem to be very susceptible to link rot - searching up the phrase is likely to bring up a working one anytime in the future, though.