I get your point, it can be really quite confusing to go from a compose file or just general instructions and mby a docker run command to the settings of truenas.

you mention jails and that’s a core (no pun intended) issue of truenas. Truenas core is based on BSD which uses jails, whilst truenas scale is based on debian and uses docker. then recently it was all combined back into one, based on linux. hence no jails, just docker.
additionally, truenas scale was using kubernetes instead of docker until a year ago i’d guess. so what im trying to say is that whatever info you may find online could be very irrelevant if it’s for truenas core or truenas scale back in the kubernetes days.

besides the ui, if you have a compose yaml and just wanna use that for setup, you can go to apps -> discover apps -> three dots next to custom app -> install via yaml. now this is a pain to find, but it is there an it works pretty well. if you hate how that editor works, you can just paste a stub there that imports a specific other yaml file and then you put all the relevant config into that one. this extra file can then be edited via the cli, copied, moved, and version controlled, which can be very convenient.

regarding storage, using ixVolumes is perfectly fine. i prefer to have a generic dataset called apps that then contains specific datasets for each app i’m running. those specific datasets i set to the apps preset.
beyond that, i’ve got more diverse setups too. for example audiobookshelf. the config and metadata storage live in an audiobookshelf dataset in the apps dataset, as described before. this apps dataset is on a small ssd pool.
the podcasts and audiobooks themselves are stored on a larger HDD mirror. basically i have a media dataset there that uses the share preset and then within that i got an audiobooks dataset that uses the apps preset. that way audiobookshelf can use the books and i can easily access the directory via smb. additionally i run a cloud sync task from the data protection tab once a week that syncs all my audiobooks to pcloud.

now all of that isn’t necessarily easy, but i find it easier and more intuitive than doing it all via the cli on debian. then again i’ve never used debian with some specialised nas ui as others have recommended.