so people have already discussed how the player is not supposed to be able to choose what exact types of creatures you summon with stuff like conjure animals and conjure minor elementals, but is the same also true for conjure elemental, summon greater demon, etc? am i able to specifically choose to summon an flail snail when i cast conjure elemental targeting an 10 ft cube of earth or will the spell just summon an earth elemental of the dm's choice? if i cast conjure greater demon, am i able to specifically summon the kinds of demons i need for my necromantic needs like the dybuuk and maurezi with this specific casting or will the spell just keep summon stupid useless demons like vrocks, shadow demons and bulezau, with me being forced to cast the spell over and over and over until i ether get a summon i want, run out of spell slots or get killed by a rampaging demon as if the spell is some kind of loot box and you are an 9 year old wasting the life savings of their parents?

Specifically Summon Greater Demon lets you chose which demon you summon:

'You choose the demon’s type, which must be one of challenge rating 5 or lower, such as a shadow demon or a barlgura.'

okay but regarding other similar spells like conjure elemental, conjure celestial and conjure fey?

Conjure Elemental, Conjure Celestial, Conjure Fey

For elemental you can influence the choice by the location in which you cast the spell, as per the description.

For the other two, it appears that the DM chooses