Ok, this is going to be a very basic, mostly simplified explanation. Very ELI5. Seems to be what's needed though.

Senators, for most of the Republican period, are former magistrates. Technically, they advise magistrates, not give orders. A senatus consultum is an opinion. However most magistrates felt compelled to respect the opinion of the senate for a number of cultural and institutional reasons. The most obvious is that one only holds a particular magistracy for a year, and then returns to the senate. Those are people you have to associate with for life.

Usually, entrance to the senate came with election to one of the magistracies of the cursus honorum.

Doubtless many senators had been military tribunes in the past, but I've never read, and I can't find now, any evidence that serving as a military tribune generally qualified one to participate in the senate. There was a period in early Roman history (as reported, at least) in which Romans experimented with Military Tribunes as executives instead of consuls, but I don't think that's what you're discussing here.

Military tribunes should absolutely not be confused with tribunes of the plebs! BTW, Plutarch is clear that tribunes of the plebs are not technically magistrates - they don't have lictors and were not elected by the populus Romanus, however over time they gained certain qualities of magistrates, such as the right to participate in the senate.

So, magistrates are elected officials who hold position for a year (usually); the senate is an advisory body of former magistrates, who while their career progresses, will probably hold other magistracies or promagistracies later on.

I have today off; I'll be around if anything needs clarifying.