documentclass{article}

usepackage[LGR,T1]{fontenc}
usepackage{textalpha}
begin{document}
φύσις
end{document

documentclass{article}

usepackage[T2A,T1]{fontenc}
% ? usepackage
begin{document}
До свидания!
end{document}

LaTeX Error: Command CYRD unavailable in encoding T1.

documentclass{article}

usepackage[T2A,T1]{fontenc}
DeclareTextSymbolDefault{CYRD}{T2A}
begin{document}
До свидания!
end{document}

LaTeX Error: Command cyro unavailable in encoding T1.

documentclass{article}

usepackage[T2A,T1]{fontenc}
DeclareTextSymbolDefault{CYRD}{T2A}
DeclareTextSymbolDefault{cyro}{T2A}
DeclareTextSymbolDefault{cyrs}{T2A}
DeclareTextSymbolDefault{cyrv}{T2A}
DeclareTextSymbolDefault{cyri}{T2A}
DeclareTextSymbolDefault{cyrd}{T2A}
DeclareTextSymbolDefault{cyra}{T2A}
DeclareTextSymbolDefault{cyrn}{T2A}
DeclareTextSymbolDefault{cyrya}{T2A}
begin{document}
До свидания!
end{document}

documentclass{article}

usepackage[T2A,T1]{fontenc}
usepackage[russian,main=english]{babel}

begin{document}
До свидания!
end{document}