the most common ways of boarding other space ships in 40k are usually Teleportation, boarding "torpedoes" (generally for Space Marines) as it's literally a torpedo with troop capacity, so it impacts hard enough that it would probably be inefficient for non Astartes, and lastly what we would consider boarding ships/pods that fit the stereotypical sci-fi brand, usually requiring fighter/bomber escorts to successfully board.

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Now as far as I am aware of in ship to ship fighting boarding is not a major thing, unless the goal is to either capture a ship, to destroy the ship via sabotage, or assassination. As such generally boarding will begin once a ship has been disabled or at the end of a battle. As it is more worthwhile to destroy a ship completely to deny the enemy than to potentially lose one yourself.

And from what I can remember from reading the Badab War FW books most fleet engagements, unless a fleet is brought to a pitch battle, would usually consist of a squadron of cruisers or a battleship and escorts, both of these would have their fair share of support craft that would be innumerable in a sense (such as no one cares how many fighters/bombers a fleet loses)

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Lastly I am only aware that if a ship had its void shields up then it is impossible to teleport onto a ship (Reference Horus and the Vengeful Spirit), the others are also unknown if they are able to pass through a void shield or their general requirements.