Trans person here. It depends on which hormone therapy you're taking.

Testosterone has a tendency to override feminizing hormones, so people who are on FtM HRT often only need to take testosterone and submit annual blood panels. But since testosterone is literally toxic in high quantities, and can cause heart palpitations, it needs to be administered carefully. Transmasc folks either go into a clinic to have it administered, or in some places they're trained on how to safely self-administer.

If you're transfemme, you have to take a cocktail of medications, since feminizing hormones have a slightly more complex response than just T. MtF people need antiandrogens, progestogens, and estrogen for their therapy. Estrogen is the real heavy lifter for most of this, but progesterone is important for things like growing tits.

People who are nonbinary (like me) may go on a lower dose of either set of hormones to achieve a more nuanced or reduced effect, depending on what type of presentation they're looking for. AFAB enbies may choose a low-dose of testosterone, or AMAB enbies may pick going on just estrogen and skip progesterone. It all depends on what you're going for.

Bottom surgery will change what medications you go on, and will make HRT a permanent maintenance medication, just like it does for cis people who have an orchiectomy/hysterectomy-oophorectomy. Sex hormones provide important chemical signals that your body needs to maintain muscle tone, bone strength, and mood. Being on the right set of hormones for you--either as a senior who is cisgender or as a trans person--has a wide array of psychological and physiological benefits. Yes, cis people have HRT, and it is still gender-affirming care for them. It's just affirming the gender they were assigned at birth. A cis woman who is post-menopausal going on HRT is just as satisfied as a trans woman taking the exact same medications.

Of course, not everyone can go on HRT, which is why transmedicalism (the idea that you're only valid as a trans person if you're pursuing medical transition like HRT and surgery) is frowned upon in most corners of the queer community. Some people have serious endocrine or cardiovascular illnesses that make HRT dangerous. Other people, especially in countries where being queer carries the death penalty, already are at risk for voicing their thoughts and feelings. They're exactly as trans as I am, whether or not they're on HRT.