An interface is a named bundle of signals. A modport is a view: which side drives valid, which side drives ready.
flowchart LR m[master] --> ifc[axi_if] ifc --> s[slave]
interface simple_if (input logic clk);
logic valid, ready;
logic [7:0] data;
modport src (output valid, data, input ready);
modport dst (input valid, data, output ready);
endinterface
Wrong vs right
Wrong — copy 20 AXI ports through every hierarchy level by hand.
Right — one interface instance, modport at each end, and a clocking/TB view later if you need it.