Wizard shcools of,
Conjuration
Necromancy
And
Druid (kind of)
(both PHB subclasses and should be in final version of BG3)
These are summoner/utility classes that summon different types of support monsters. Conjuration mages summon more powerful animals, elementals or undead that gain power with spell level. Necromancers are a bit different as they heal from spell attacks that kill enemies (grim harvest at lv2) and can summon more skelemans or zombies increasing in power/number depending on level (2 skelemans/zombies at lv 6 rather than 1). Druids can summon fey (conjure woodland beings), animals and elementals but they are more mage/clerics.
The issue is you need to be level 6 with pretty much any class to summon anything other than a familiar. Ritual summons like ranger and warlock are just decent familiars. Familiars of wizards and sorcerers are supposed to be able to cast the touch spells of the wizard/sorc remotely. At level 8 you get 4th level spells one of which is summon lesser elementals and enables you to summon 8 x 1/4 CR or 1 x 2 CR elementals increasing with spell level.
This is all in the PHB and is a free PDF download and well worth a read. There are simply too many spells, items and potential variations to cover without making mistakes in translation I am not experienced DM. Not played D&D TT since 3.5e long time ago.
