Palworld

Firstly you need to reach lv 33 to unlock the Summoning Altar in your Technology tree (requires 3 Ancient Technology Points to unlock and 100 stone and 20 Paldium Fragments to build).

Then you need to collect 4 Bellanoir's Slab Fragment (and turn them into Bellanoir's Slab (can be done at a Production Assembly Line).

From what I have gathered Bellanoir's Slab Fragment can be obtained from treasure chests in Dungeons, Wildlife Sanctuary islands, or inside Mineshafts (these are the places I have found them in so far). Dungeon chests have had highest drop rate for me (14 fragments found in 20 dungeon runs).
Note that I have only done lv 13 dungeons (which take around 2 min to complete if you know the layout) and don't know the drop rate of higher vl dungeons.

Once you have done this you can summon Bellanoir at the Summoning Altar. Once you have summoned Bellanoir the Summoning Altar will be destroyed.

The Summoning Altar can be build within your base so you can use all your Pals assigned to that base in the fight.
The Bellanoir boss is level 30 and has 294K hp, so I would recommend to level all the pals within your base to at least that level before you summon.

The Bellanoir boss can't be captured, but once you have defeated Bellanoir you will get an egg that will hatch a Bellanoir of your own.

I will try and keep updating as I gather more information.