Like with RCS, you just press to get the seven reactions, which are unchanged. If the SMS recipient is using Messages, the reaction just appears in the bottom-right corner.
For recipients that are not using the Google Messages app, don’t have RCS chat enabled, or on iOS, the fallback is a text message with the emoji from the sender followed by what’s being reacted to in quotes.L: Messages app leaving reaction on SMS | R: SMS reactions from Android on iPhone (1st, 3rd, & 4th lines).Apple takes credit for iMessage reactions on Android