IPA symbols are approximate, representing regions of articulation rather than points. For example, if a consonant has the features voiced, bilabial, oral, and plosive, the symbol [b] can be used. This may or may not correspond with the conventional symbol used for the given phoneme in a given language. As it happens, [b] and /b/ are used by convention in both English and Russian. However, this does not imply that the range of allophones for /b/ in the two languages are the same.
The most important distinction between Russian б and English b is voicing. In English the main contrast between /p/ and /b/ is aspiration: /p/ is aspirated and /b/ is not. For this reason, English /b/ is not always fully voiced, as it is the contrast of the aspiration feature, not the voicing feature, that English speakers primarily use. In Russian, there is no aspiration of /p/, and /b/ is fully voiced in contrast. To an English speaker, this can make it sound stronger, heavier, ... (pick your own subjective description). Regarding your comment about buildup of air, it may also sound that some preparation of voice is happening, compared with what you expect in English, because again the segment is fully voiced from the beginning, whereas voice onset time is delayed for /b/ in English.
As you correctly point out, Russian /b/ is also pronounced with somewhat more lip rounding than in English.
Regarding your comment on implosives: I am not an expert but have never heard of implosive rendering of Russian /b/. If you don't have a good reference but are only guessing, I advise you to abandon this guess.
