Predominantly through the airport in Chișinău, the capital of Moldova, although steps have been taken to reduce the number of troops that require rotation by employing Transnistrian citizens with Russian passports.
According to a January 2022 report from the Friedrich Ebert Foundation, rotation is achieved by unofficially switching the assignments of Russian peacekeeping troops stationed in Moldova as a condition of the 1992 peace agreement.
The Russian military deployed on the territory of the Republic of
Moldova is grouped into two categories: the Russian “peacekeepers”
which should maintain the peace in the region since 1992, since the
end of the Dniester War, and the military of the Russian Troops Task
Force (GOTR). The latter, officially, have the mission of guarding the
ammunition depots in the village of Cobasna, which housed about 20,000
tons of ammunition in the early 1990s. The depot is located a few
kilometres from the Ukrainian border and is considered to pose a real
danger to the region.
While the Russian “peacekeepers” are mandated on
the territory of the Republic of Moldova, GOTR has no legal reason for
its presence in the Transnistrian separatist region. Moreover, the de
facto “pacifiers” and GOTR are one and the same force that only
performs periodic rotation manoeuvres. Thus the “peacemakers” become
GOTR soldiers and vice versa. In fact, Russia does not even bring its
own soldiers, but rather, for years, has been recruiting
Transnistrians. This is not even very difficult, given that more than
220,000 citizens of the Transnistrian separatist region also have
Russian passports, accounting for about two-thirds of the total
population on the left bank of the Dniester.
This echos some of the claims made in a 2016 report by the European Parliament's Directorate-General for External Policies:
Since 2014 Russian troops are no longer able to transit through Ukraine
to Transnistria. Troop rotation takes place through the airport in
Chisinau, which increases the Moldovan authorities’ influence on the
presence of Russian soldiers. Thus, in order to maintain staffing
levels of military units, Russia recruits more and more Transnistrian
citizens with Russian passports.
Both reports also mention that since 2014, the OGRF/GOTR have increasingly relied on Transnistrian citizens holding Russian passports for recruitment, reducing the reliance on troop rotation from Russia. A 2020 report by the Robert Lansing Institute notes that "the representative of the TMR in the leadership of the JCC for peacekeeping operations, Oleg Belyakov, said that RTF will be fully staffed by Russian citizens residing in the republic."
With regard to supplies, a December 2016 report by the Swedish Defence Research Agency claims that these also transit through Moldovan territory after arriving in Chisinau by air. It notes also that limited troop rotation takes place through the airport:
After the Ukrainian parliament’s decision, in May 2015, to suspend all
military cooperation with Russia, which also included cancelling
military transit rights through the Odesa region to Transnistria,
Russia was forced to transfer supplies by air to its base, through
Moldovan territory. Although this has given the Moldovan authorities
some more leverage in controlling the rotation of the Russian
peacekeeping troops at the Chisinau International Airport, it has also
led to some dissatisfaction with the Russian declarations. For
instance, there have been cases where people have been entering as
tourists, but then turned out to be a commander of a military unit.