Russian military service legal framework is somewhat of a moving target in the recent years, but here's what we have:
Since at least Soviet times, Russian men, when turning 18 (as well as women of military significance, e.g. doctors, nurses, etc) are required to register at the local conscription office. Registration is to be updated in regard to place of residence, education, health conditions, etc... until ~65y of age. Naturalised citizens are expected to do the same. The registration makes one available for both draft and (should a need arise) mobilisation.
The point above was profoundly disregarded since 1990s and well until the end of 2022. No one cared and no one enforced the requirement en masse (targeted enforcement was indeed a practice).
There is 1-year obligatory military service for men that is expected to start between 18 and 30 (27 up to 1 January 2024) years of age, subject to postponing for variety of reasons (e.g. university attendance or medical conditions). Because of (2) and a variety of other factors, less than half of the men who would be considered fit really did serve. (Note: A fulfilled military obligation is a requirement for a lot of career paths in Russia.)
Things changed a lot in late 2022 and the enforcement for both registration and service became gradually stronger.
One of the new enforcement measures is that naturalised Russians became subject to removal of Russian citizenship for a failure to register at the conscription office. Media outlets are frequent to depict police raids at places where migrants live, work, worship or socialise. Those caught without a valid military registration are either forced to register or deported to their country of origin with their Russian citizenship removed.
Russians by birth are not stripped of citizenship, this has no legal basis (as of now) and would be counter-productive in regard to making them available for draft or mobilisation.