Maybe I can help a little bit – As you know from Rose’s B12 site and many posts throughout the forum Vitamin B12 has multiple forms including cyano-, hydroxo-, adenosyl- or 5’-deoxyadenosyl-, and methylcobalamin. The liver does not convert sufficient amounts of cyanocobalamin (the most common form) to therapeutically useful levels of methylcobalamin, the active form found in the CNS. Among the many benefits of methylcobalamin it helps lower plasma levels of homocysteine (Hcy) and is also key for the synthesis of S-adenosylmethionine (SAM) and it is critical for nerve health, repair, and regeneration. Up to ~90% of methylcobalamin can be absorbed when taken sublingually although absorption also depends on levels of your intrinsic co-factor as well. I'll let other forum members more well versed than I am elaborate further.

As to your ? ~1mg B12 pills - I think that since the daily requirements are listed as mcg the suppliers list the strengths in mcg as well. However, the conversions are very easy and should help somewhat
1000ug or mcg (microgram) = 1mg (miligram)
1000mg = 1g (gram)
1000g = 1Kg (kilogram)
1000Kg = 1MT (metric tonne) - a lot
You can really get carried away with this since
1000pcg (picogram) = 1ng
1000ng (nanogram) = 1ug or mcg

Hope this helps not hinders.
Alkymst